Almost since the moment the planes hit, the "9/11 Truth Movement" has been active in trying to argue that either the US (or Israeli) governments plan the attacks or had foreknowledge of the attacks. The most obnoxious truthers are those who claim that the buildings were not brought down by hijacked airplanes but rather by missiles or explosives in the buildings.
Just on its face, the argument is ridiculous. Looking at how Iraq has been screwed up by the administration, the truthers suggest a level of competence that our government has not shown. Furthermore, bin Laden and al-Qaida has stated that they did the attacks. In the latest tape, bin Laden reads what he claims was the last testament of one of the hijackers. Granted one can argue that the tapes are fakes and given the strange issues of the beard and the "freezing" of video, I am maybe reconsidering my previous statement that bin Laden is alive, and open to him being dead.
Still, it is fairly certain that the tapes were produced by al-Qaida. Yet the truthers will soldier on.