There were a lot of comments. Long comments. I have tried to summarize the anticipated possible attacks below. 

Pipes indicates multiple submissions with the same or similar method.

Anticipated Methods:

Terrorists might sink oil tankers with speedboat bombs
Terrorists might poison water reservoirs |||
Terrorists might do a gas attack in subway
Terrorists might detonate bombs in subway
Terrorists might destroy irrigation equipment
Terrorists might disrupt elections with polling place snipers
Terrorists might introduce mad cow with tainted organs
Terrorists might attach bombs to skyscraper support columns
Terrorists might fire rockets towards the white house from a van roof
Terrorists might send pretzels to the president
Terrorists might release a phony government announcement to ban alcohol
Terrorists might start lots of fires |||||||||
Terrorists might unplug ticker tape thing
Terrorists might challenge the president to a duel
Terrorists might install a giant gasoline reservoir inside a large cargo van
Terrorists might resurrect Steve Guttenberg's career
Terrorists might poison Starbucks Coffee |||
Terrorists might dust fabric industry with itching powder
Terrorists might give president of USA a cat
Terrorists might shoot natural gas storage tanks and trucks with armor-piercing bullets

Terrorists might destroy the volcano at La Palma |||||
Terrorists might detonate a subterranean nuclear weapon
Terrorists might deflate the Super Bowl footballs
Terrorists might destroy a Soo Lock ship lock by lowering a bomb from a fishing boat
Terrorists might do 50 simultaneous mall bombings
Terrorists might dust America's corn with pollen from genetically engineered poisonous corn strains
Terrorists might expose Americans to osmium
Terrorists might drop deadly castor beans into the three-bean salad at salad bars
Terrorists might trick people into entanglement with the lethal Texas penal system
Terrorists might 1-2 punch suicide bombers strike the white house
Terrorists might hijackattack a jet into a football stadium
Terrorists might wrong-way freeway tractor-trailer trucking
Terrorists might overload the WalMart express checkout lane by writing a check
Terrorists might join the National Guard, become a pilot and strafe schools.
Terrorists might elect Bush ||||||||
Terrorists might park and burn barges of sugar and nitrates near critical river-side buildings
Terrorists might construct a 1/4 scale Remote-controlled ducted-fan f-16, filled with explosives and strike Washington
Terrorists might taint baby food with cat poop
Terrorists might Bomb Union Station with a suitcase bomb
Terrorists might push poison into an urban water supply with a basement pool pump
Terrorists might exploit diabetic security loophole to hijackattack airliners with poisoned hypodermic needles
Terrorists might frame a supposed antagonistic country by attacking with a doctored hijackattack

Terrorists might mix superglue with women's makeup
Terrorists might drive a bomb-laden remote-control van into a busy casino lobby
Terrorists might create and spread a dehabilitating computer virus
Terrorists might put poison in a water tower
Terrorists might breed and propogate killer locusts
Terrorists might steal and use US fighter jets from Air Force Bases
Terrorists might irradiate Wall Street or California's Central Valley with a radioactive bomb.
Terrorists might send smallpox or tb infected terrorists on a tour of American airports and casinos
Terrorists might poison the water misters at Disneyland
Terrorists might poison personal misters at ballpark giveaways
Terrorists might drop grenades into a crowded sports stadium
Terrorists might poison all the meals at a crowded restaurant
Terrorists might incite trampling in crowded churches
Terrorists might become knifemen attacking malls in a coordinated manner
Terrorists might try hurling stuff off overpasses
Terrorists might taint valentine roses with anthrax
Terrorists might build a ramp to jump a truck into a crowded theatre
Terrorists might steal numerous port-a-potties, and place them in front of the Whitehouse
Terrorists might switch people's mail around
Terrorists might crash oil trucks on freeway overpasses
Terrorists might Shoot the ultra-high tension power transformers

Terrorists might Destroy the large natural gas mains under the Mississippi River
Terrorists might embrittle a bunch of airplanes with leaky mercury packages ||
Terrorists might seize offshore drilling rigs in the Gulf
Terrorists might overtake the public address system at a sporting event
Terrorists might use shipping container bombs to sink container ships, clogging major ports
Terrorists might poison communion at rural churches
Terrorists might machine gun congregations at rural churches
Terrorists might dump a bag of broken light bulbs over Times Square on New Years
Terrorists might poison drinking straws at the factory
Terrorists might create a hostage situation at the NYSE or other exchange

Terrorists might spread anthrax in the subway
Terrorists might force them to eat horrible food
Terrorists might start pencil-poking
Terrorists might kidnap Alan Greenspan
Terrorists might poison beer and make it explode
Terrorists might hijack the Emergency Broadcast System and spread false information
Terrorists might coordinate a 50 state citizen sharpshooting spree
Terrorists might destroy the four east-west highway arteries simultaneously
Terrorists might poison bulk food bins
Terrorists might do suicide bombers in Walmart
Terrorists might so a suicide bomber on stage for American Idol finale
Terrorists might promote catfights
Terrorists might do simultaneous 'wardrobe failures' 

Terrorists might do 50 state coordinated mall shootings
Terrorists might create an army of Jesus clones
Terrorists might do a poison gas attack in forced ventilation skyscrapers
Terrorists might poison food at a food processing plant
Terrorists might attack a crowded stadium with toxic gases in the air conditioning
Terrorists might poison McDonalds restaurants in many places at once
Terrorists might do a gas attack crowded WalMarts
Terrorists might hijackattack w/a helicopter
Terrorists might send packages with white powder
Terrorists might pose as a bus driver, drive full bus off of a cliff
Terrorists might choreograph a head-on train collision in a city
Terrorists might poison one product, send letters indicating that many products are poisoned

Terrorists might cause car accidents by seeding freeways with nails
Terrorists might dirty nuke briefcase bombs at the Capitol building
Terrorists might blow up a railroad bridge while a chemical train is crossing
Terrorists might cyberattack the computer systems of many powerplants 
Terrorists might do multiple assassinations of local and state figures 
Terrorists might train millions of rabid, disease infested bunnys to attack the US
Terrorists might delete very important documents in US corporate headquarters
Terrorists might hijack and destroy a large oil tanker ship in port

Terrorists might spread a biological agent at a large airport hub ||
Terrorists might grenade or machine-gun a crowded movie theatre on an opening night 
Terrorists might do 50 state coordinated killers
Terrorists might take over testing facility that tests water, poison water
Terrorists might use thermite to destroy the support beams under the Empire State Building
Terrorists might storming the white house by use of 200 well armed and defended terrorists
Terrorists might fill a burlap sack with bullets, climb a tall building, and throw them off in handfuls.
Terrorists might do 500 coordinated attacks in the heartland
Terrorists might infect people on a few international flights with smallpox
Terrorists might use mad cow disease swabbed into the noses of cattle ||
Terrorists might detonate a nuke in a storage container 
Terrorists might place a bomb under a table at the international scrabble championship in new Orleans 
Terrorists might poison the food at America's Foodcourts
Terrorists might infect our paper cash with some sort of chemical or biological agent
Terrorists might bulldoze the bleachers at a sports arena
Terrorists might do a series of attacks on small, unguarded government buildings in small towns

Terrorists might use a potato gun to launch C4 explosives at the white house
Terrorists might use paint remover to deface museum paintings
Terrorists might isolate and fire bomb a theatre or restaurant ||
Terrorists might plant land mines in America
Terrorists might become snipers ||
Terrorists might write a computer program to cut off all the power in the United States
Terrorists might cut off the coffee supply
Terrorists might assassinate minor Govt. officials: mayors, governors, senators and representatives.
Terrorists might plant bike path land mines
Terrorists might many small explosives spread over an area of a power grid to create massive blackout.
Terrorists might use the Hubble Space Telescope to create a thermal disruption in the World's oceans, killing enough Phytoplankton to disrupt world oxygen supplies.
Terrorists might go on a state fair killing spree
Terrorists might equip rabbits with explosive-laden backpacks, with detonators attached to their heart-rate monitors
Terrorists might poison an escalator banister.
Terrorists might isolate Seattle by attacking its transportation routes.

Terrorists might poison bottled water factories
Terrorists might explode bombs at dozens of shopping malls nationwide
Terrorists might random bombs at polling places on election day
Terrorists might boat-bomb a cruise ship far from help
Terrorists might contaminate Red, White and Blue Dyes with Anthrax, then use a small attack to boost patriotism.
Terrorists might pull up a railroad rail here and there
Terrorists might use two armored machinegun attackers at a Halloween street festival
Terrorists might attack a football stadium crowd in an attack helicopter
Terrorists might try contamination of over-the-counter drugs
Terrorists might attack a meeting hall with only two exits
Terrorists might poison a large eating event
Terrorists might poison water reservoirs with mtbe
Terrorists might infect a mall with weaponized Soviet smallpox virus
Terrorists might taint a slaughterhouse feed bin with BSE and wait, then inform the media
Terrorists might snipe the pilot or crewmember of an airplane while it is docked at a gate
Terrorists might detonate the anchors of the cables on one end of a suspension bridge
Terrorists might conceal a large bomb on a plane by shipping a concealed package
Terrorists might exploit widespread fears of a "dirty bomb" with an only slightly radioactive explosion.
Terrorists might use chemicals or sharp objects to destroy priceless art treasures
Terrorists might park gasoline tankers on railroad crossings
Terrorists might bulk-mail millions of small incendiary devices
Terrorists might destroy the 2 aqueducts providing all of Manhattan's potable water
Terrorists might purchase the taco-bell chain and offer a "20 tacos for a buck" deal
Terrorists might infect the soda machines at a stadium with botchulism
Terrorists might unleash a cotillion of angry Walmart greeters

Terrorists might walk in and lay waste to hundreds of people in the mall
Terrorists might inject AIDS-tainted blood into Pepsi products
Terrorists might create automobile accidents simultaneously around the country on major roads, at rush hour
Terrorists might plant two land mines in Central Park, New York
Terrorists might go on a shooting rampage in movie theaters 
Terrorists might steal North Dakota 
Terrorists might poison Pixy Stix
Terrorists might hold hostage the patrons of a movie theater on a crowded opening weekend
Terrorists might do nothing, just leave it as it is, it's self destructing anyway.
Terrorists might clone dinosaurs and let them loose
Terrorists might promote the Atkins Diet Plan
Terrorists might dump trillions of dollars perfect quality counterfeit cash in an air drop across the US
Terrorists might activate an electronic satellite jamming system
Terrorists might give away ipods with pre-recorded brainwashing messages
Terrorists might use rat poison on ATM deposit envelopes
Terrorists might break Florida off of the United States. That little state looks so brittle.
Terrorists might use rat poison to poison water reservoir
Terrorists might attack the US with one of the former USSR's many unsecured nukes
Terrorists might steal explosives at a mining company and attack small towns.
Terrorists might try a mass car hijacking, involving suicide collisions in 100 stolen cars
Terrorists might shake it like a Polaroid picture
Terrorists might inject quantities of smallpox, thallium, anthrax etc. into the water supply
Terrorists might use bombs to destroy a large prominent college or university
Terrorists might visit and attack West Point with a weekend car bomb
Terrorists might simply kill as many people as possible, in the manner of serial killers, but with the marks of terrorist groups.

Terrorists might replace the egg-free cookie dough in B&J's Cookie Dough ice cream with cookie dough containing eggs
Terrorists might sink a barge loaded with a super-absorbent polymer, damming the river
Terrorists might sabotage and sink a cruise liner
Terrorists might resource SoCal water supplies from Huntington Beach's ocean water
Terrorists might take hostages and force them to watch 10 minutes of the Master of Disguise.
Terrorists might plant explosives on those traffic light boxes. 
Terrorists might go to the moon and implant gigantic rocket propellers on it to make it crash on Delaware
Terrorists might bomb several main electric transmission towers in many service territories
Terrorists might fill phony Santa Claus belly with plastic explosives, detonate while children ask for toys
Terrorists might Simultaneously unleash hundreds of Petanque balls made from Smoke Detector Americium-241
Terrorists might hack Cockeyed.com and destroy its contents
Terrorists might use a fertilizer truck bomb a major bridge
Terrorists might undermine the foundations of nuclear power plants by injecting large amounts of water
Terrorists might launch small shoulder mounted missiles at airport airplanes
Terrorists might weld steel wood splitting wedges to multiple railroad tracks
Terrorists might blow up a truck while driving over a bridge at the mouth of a busy port.
Terrorists might infect bubble wrap with infectious agents as it is being produced.
Terrorists might call in a four-bomb threat on a three-bomb city.

Terrorists might detonate an ink bomb, with the type used in clothing security tags, in a museum
Terrorists might poison a water supply with Fluoride
Terrorists might explode a ship full of explosives at the Panama Canal
Terrorists might use women and virus-laden perfume bottles to spritz-infect a sports arena at gametime.
Terrorists might create a tree-planting campaign which creates excess oxygen and prematurely oxidizes the Statue of Liberty
Terrorists might strand vehicles at railroad crossings
Terrorists might detonate a dirty bomb in a shipping container
Terrorists might bring down the Internet and all connected computers with a virulent computer virus
Terrorists might steal a backpack full of library books
Terrorists might cause a small plane crash into a sold out college football stadium
Terrorists might crash a plane at the intersection of I-75 and I-80 
Terrorists might light the leaves in our gutter with gasoline
Terrorists might blow the Golden Gate bridge with a rental truck full of explosives
Terrorists might increase laundry soap nationwide creating massive sudstorm.
Terrorists might blow the aqueduct feeding LA and divert into the Salton Sea
Terrorists might blow the control structure preventing the Red River from capturing the Mississippi
Terrorists might limit the flu vaccine supply 
Terrorists might trick us into spending all our money on a couple terrorists rather than education and healthcare
Terrorists might use a series of mid-column mines directed to intake of large hydroelectric plant

Terrorists might create the bra-bomb, and panty bomb to go with the shoe bomb
Terrorists might work to slowly erode the racial harmony that makes our country strong
Terrorists might overload the 911 emergency system
Terrorists might remove sections of rail from many railroad tracks nationwide
Terrorists might blow all of the bridges and tunnels into and from the island of Manhattan
Terrorists might purchase and ignite duty-free high-proof vodka in flight
Terrorists might roil the US/Mexican relationship
Terrorists might try a superbowl Halftime Blimp attack
Terrorists might truck-bomb attack Walmart during Thanksgiving sales
Terrorists might use a multi-truck bomb a college town
Terrorists might rat poison the water supply
Terrorists might poison the pet food supply
Terrorists might infect water supply with AIDS virus
Terrorists might pack a life-like robot panda with explosives, kill other pandas
Terrorists might attack the leaders with a gun-in-a-videocamera during a press conference

Terrorists might switch flu vaccines with smallpox injections
Terrorists might sabotage traffic-light software
Terrorists might mill collapsing steel bridge girders and cables
Terrorists might release attack monkeys
Terrorists might buy a bus company, replace drivers, massive simul-crash.
Terrorists might taint water supply with LSD
Terrorists might derail trains with machined iron traps
Terrorists might park a dirty nuke RV in the infield area at the Daytona 500
Terrorists might ignite a gas station
Terrorists might park cars on the train tracks ||
Terrorists might taint a shipment of flu vaccine 
Terrorists might shoot electrical transformers 
Terrorists might walk a briefcase bomb onto the trading floor at Wall St.
Terrorists might use widespread but focused snipering on a select population of Americans
Terrorists might use a C4 Winnebago attack Mass Avenue in Boston under the Prudential center
Terrorists might crash fertilizer-bomb Cessna airplane into building after dumping poison on area
Terrorists might poison a food additive that is used to produce a variety of food items
Terrorists might car bomb a southern county fair
Terrorists might try cross-country coordinated truck-bombs on bridges
Terrorists might explode gym-bag bombs in busses
Terrorists might exploding binoculars for college football games
Terrorists might destroy high capacity power lines in rural areas
Terrorists might poison the supply of methamphetamines in the US
Terrorists might unscrew the tops on ALL salt shakers at public restaurants on the same day
Terrorists might rob Ft. Knox with the help of ultra hot aerobatic flying team pilot-girls 

Terrorists might convince the corn crops not to mature
Terrorists might throw the nation's largest pancake breakfast
Terrorists might multi-mall truck bomb attacks near Thanksgiving
Terrorists might suicide bomb a sporting event
Terrorists might kill the head of every fortune 500 company when he went out to get his morning paper
Terrorists might use model rockets to transport copper wire onto high-voltage power transformers, grounding them.
Terrorists might destroy railroad tracks with a rented forklift
Terrorists might infect the bills and coins of the US with disease or poison
Terrorists might remove all the pictograph buttons from fast food cash registers
Terrorists might set off a nuclear powerplant
Terrorists might cover a location with tons of thermite
Terrorists might bury all the gold in Ft. Knox
Terrorists might hijack a Sausage & Cheese cart and slowly destroy the economy of the mall
Terrorists might reveal the secret societies of the Skulls "secret handshakes". 
Terrorists might rig all the bridges over the Potomac with explosives and detonate them during rush hour.
Terrorists might Uhaul truck bomb Wall Street
Terrorists might bring down the internet forcing nerds to find more constructive pursuits.
Terrorists might fly a jumbo jet into a nuclear reprocessing facility
Terrorists might build a pretzel catapult to assassinate the president
Terrorists might take over the emergency broadcasting system and replace our television programming with a large, hypnotizing wheel.

Terrorists might truck bomb bridges to NYC, stifling food delivery ||
Terrorists might try clandestine spraying of anthrax on fresh produce in supermarkets in multiple cities
Terrorists might drop Ricin-laced clothing in donation bins at Goodwill
Terrorists might poison McDonalds beef products, with Mad Cow.
Terrorists might poison on millions of acres of farmland, destroying the crops and making the soil worthless
Terrorists might poison soda supply at McDonalds with LSD
Terrorists might spread smallpox across many air-hubs on a travel-spree
Terrorists might hack whitehouse.org to point to cockeyed.com
Terrorists might call in bomb/terrorism scares at major train stations and airports
Terrorists might bring the kidnappings and beheading videos from Iraq to our home turf
Terrorists might boost the alcohol content in Budweiser to 80%
Terrorists might try a nerve-gas attack in an enclosed football stadium
Terrorists might fill Wall Street with jello
Terrorists might simultaneously remove the airports in Chicago (O'Hare) Dallas/Ft. Worth and Atlanta
Terrorists might poison reservoirs
Terrorists might use Lexan plastic weapons to hijack-attack more skyscrapers
Terrorists might dynamite attack natural gas pipelines in the US.
Terrorists might drive your Chevy to the levee filled with TNT
Terrorists might well they could steal all the doormats and use them to clog the drains, flooding the country.
Terrorists might release trained, highly destructive monkeys onto any large, commercial building. 
Terrorists might rolled a tanker truck over right under an overpass, taking out two highways
Terrorists might open and supply a maximum security prison, freeing and arming hundreds of escaping felons
Terrorists might sabotage an American Car company assembly line such than the cars will explode
Terrorists might slowly but surely damage American highways in a fleet of terrorist trucks.
Terrorists might Plant gen-joked seeds to breed anti-depressant corn crops across the US.
Terrorists might Use lifesize plastic explosive sculptures of homeless people in cardboard shantys beneath the Alaskan Way Viaduct in Seattle.
Terrorists might fill up a hi-rise hotel room with explosives 
Terrorists might soil a skyscraper with mercury waste