This wasn't that hard of a act to commit (once you get past the morals part) and there can't be any real protection against it. It seems that the thing that kept the plane that crashed in PA from hitting it's target was that the passengers and/or crew onboard fought back. The couldn't save themselves, but we'll never know how many people that plane would have killed. You cannot effectively fight someone who is 100% sure that they are correct, and who is willing to die. The most frightening thing of all it that this could very well have been just a small group from anywhere whose was not sponsered by any nation, and who we may never be able to punish. Only time will tell.
-------------- Tony Zahn Sofware Engineer, Achievement Technologies Inc. Go Blue!