It would be too difficult to happen again from the terrorist point of view. First of all getting a team together who are all most likely already known by the CIA, getting into the US, potentially taking plane lessons in the US [without causing suspicion]. Then there's getting anything that could cause harm on the plane, since airport security I've heard is ridiculously insane in the US. Then there's getting into the cockpit, which since 9/11 are always locked, bullet-proof, one pilot has to be in at all times, etc. And finally, the public are so much more aware of what is happening now. During 9/11 planes had NEVER been hijacked and used as weapons, so the passengers never thought that's what was happening. Everyone now instantly jumps to "9/11 OMG", and wouldn't be submissive with the terrorists.
