Darn, i was just about to bring this up.
I was watching Bowling for Columbine and decided to rewatch the WTC attack. (I felt like being a liberal today )
Anyway, I was watching and I noticed the planes didn't hit the center of the buildings, they hit near the top. (Where I assume the buildings were weaker.)
I can only assume that it wasn't only the heat that caused the first tower to fall, but the combination of the force of the planes hitting the skyscraper, (Even wind has been known to cause skyscrapers to sway), the weakening of the metal from the heat, and the weight of the top part of the tower that caused the top to fall onto the rest of the tower.
And of course, this started the domino effect that destroyed the rest of the area.
Not to mention that, for all we know, there may have been explosives on the planes themselves.
Of course, as I've said before, I'm not a scientist, nor am I even educated (I'm working on it!) but in my opinion, whether the U.S. Goverment had a part or not, two 747s had enough power in themselves to destroy the tower.