you could even culture bacteria like plague. get some antibiotics, and expose the bacteria to low level antibiotics of all known types. before long you would breed some plague bacteria resistant to all known antibiotics. put it in a spray bottle, and give massive numbers of people a nice and highly lethal infection in the lungs. Slightly worse then anthrax, and communicable from person to person without fleas if lung infection occurs.
Or you could bomb the cooling pond of fuel rods at any university that has a reactor. All fuel rods must be stored on site in america due to federal law. And all universities with nuclear reactors are in densely populated areas. (There's one at the U of U here in Salt Lake City. Those who work there call it the Deep Fryer) Think of the long term contamination of a large city and what that would do. You could find a nice gasoline tanker in the center of bumper to bumper rush hour traffic and hit it with a RPG.... Or just shoot a tank filled with silane for semiconductor manufacture. There's a huge volume of highly toxic chemicals transported around the nation for industrial use. Simply break the container and you have a major problem. The number of potential targets is near limitless. While the number of people available to police things quitelimited by comparison.
Given that people are STILL getting onto aircraft with bombs, post 9-11. Despite all the security measures, I think it's safe to say that the security is FAR from perfect. Considering the failure to perfect security around a single target: the airports, I have no hopes whatsoever that it's remotely possible to protect people against every possible attack. There's just too many targets, too many potential weapons, and too many people to watch everything and everyone. And if you could, all you would have is the perfect police state. (Nazi party anyone?)
I don't think the benefits outweigh the downsides. I would rather not walk through a full body scanner and take a car instead, then go through an airport with one. But I would freely fly anywhere regardless of terrorist risk. The odds of dieing to a terrorist attack are less then 1 in 1 million. Eventually the odds of being virtually strip searched at an airport will reach 100%. I personally would rather take my chances with the 1 in 1 million, but then I value my privacy a little excessively. I guess just a personal quirk.
At any rate, it remains logistically impossible to protect everyone against every threat. To protect everyone you would have to be able to control everyone. I can't imagine a greater threat then a serious attempt to do just that. IMHO the level of security at the airports is already proof they are winning the battle to make people afraid, and we are losing. I already have to take off my shoes to go through an airport after one person hid a bomb in one. Just wait till someone hides a balloon filled with a liquid explosive up his ass to get past a full body scanner. "Ok now bend over and cough......."