It makes perfect sense to a tinfoil hat wearing moron, however Snopes shoots it full of holes.
http://www.snopes.com/rumors/pentagon.htm
If that isn't good enough, here is a link to the actual pictures from inside the Pentagon with photos of the plane wreckage.
http://rense.com/general32/phot.htm
By the way, one of my former Commanding Officers was killed by that 757.
http://www.arlingtoncemetery.net/ldgetzfred.htm
Fuk those guys, and fuk anyone who makes jokes about it.
Tom