Which is Intelligence and airline security are bound to fail. Intelligence is - and will always be - imperfect in collection, and subject to poisoning by the adversary. Air travel is a porous target - repeated "penetration tests" show a 100% success rate in compromising the system. Look, any system that has to move several hundred million passengers a year is by necessity going to have all sorts of failure points.
Could the international Intel agencies have done better? Sure. Could TSA improve security and lessen hassles? Sure. Can either move the failure rate down significantly? I don't think so.
As long as you play defense, you're going to lose. If your adversary is clever, and patient, and well funded, he'll get you. He can study your defenses, pick a vulnerable target, and plan an attack to bypass your counter measures. In the long run, he'll win.
It will be a long, long time before the Intel archives of the last decade are declassified. In the cold light of day, it will be clear that a lot of the "thwarted" terror attacks were nothing of the sort. What will be clear is that Bush's decision to go on the attack - to take the battle to the Arab heartland where Al Qaeda had to respond, or lose relevancy to its intended supporters - was the decisive moment. Bush didn't "take his eye off the ball" by going to Iraq, he forced Al Qaeda to fight our battle, rather than them forcing us to fight theirs. Their resources went to operations other than attacking airliners.
It remains to be seen whether the Democrat's desire to withdraw from Bush's aggressive overseas engagements will put us on the defensive; certainly they want us on the defensive. I fear that Al Qaeda has been hurt, but not fatally.
Defense always loses in the long run. If we play defense, it's not a question of "if" - only "when" and "how".
UPDATE 28 December 2009 11:07: Unbeknownst to most people, there exists a set of immutable Laws of Security. Marcus Ranum described the first - and greatest - of these. Sometimes it's easier to not do something stupid than it is to do something smart. The TSA is fixin' to destroy that