Terrorists always change tactics to keep us off guard. They may not be able to hijack anymore, but they can sure as hell still put a bomb in the cargo hold - especially if they're flying out of a smaller airport with old equipment (or even a larger airport staffed with rent-a-cops manning the x-ray machines). We also know that it's incredibly easy to get a job that provides free access to aircraft (food service, security, etc.), so someone could potentially even hide a machine gun somewhere on a plane if they really wanted to. I've seen enough hidden-camera investigative reports where undercover reporters basically did just that to not have it in the back of my mind. Especially when you hear things like 700 ID badges and a couple dozen uniforms being stolen from an airport (forget which) a while back.
Don't mean to be alarmist, but I don't think anyone should have a false sense of security right now. The exact circumstances of 9/11 could probably not happen again, but other terrorist acts that could be just as bad surely could still happen. Likely? Obviously not on any given flight, but definitely possible.
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!