A number of people have pointed out that this particular attack technique will only work once.

All evidence points to the fact that none of the crew or passengers on the airplanes knew until the last moment (if then) that their planes were on suicide missions. The terrorists were almost certainly flying the planes, having removed the original pilots. At least one of the airplanes that flew into the WTC had only a few minutes beforehand declared an intent to land at JFK airport.

Now that everyone knows what happened, any terrorist that tries to commandeer an airplane in the future may have to kill everyone on board, since the crew and even the passengers will now fight like trapped rats. This is not something that can be accomplished with a cardboard cutter or can of Mace.

No, they'll have to think of something else now. Unfortunately, they probably will, unless we stop them once and for all.