Also an interesting piece of napkin math calculating the kiloton yield equivalent of the WTC collisions and collapse. Hmmm, 1.9 kilotons and 3.5 kilotons respectively? Just for comparison, here’s a 1 kiloton fission detonation from the nuclear tests of the early 50’s.
Update: The ever-reliable Snopes have a definitive debunk of the Pentagon conspiracy theories, and Popular Mechanics has a full 9/11 myth-debunking section. Also see 911Myths.com.