I am one of these so-called dumb friends of Kman. I think he's placing far to much emphasis on our arm argument, but to me that's only a slight edge to the Allosaurus. It's the quick speed of the more nimble allosaurus and its incredible jaws, which could expand like a snake's and take 100 pound chunks out of another dinosaur, that make it the better.
The sheer size, as it was put by Kman, of the T. Rex really matters not in my opinion. Its slow reflexes could do nothing to avoid the severe wounds that the allosaurus could make. Also, these slow reflexes prevent the massive carnisaur from overwhelming the allosaur with its own attacks. The simplest way to put my reasoning is that the allosaur can attack many times without retaliation from a slower T. Rex.
The way I imagine the fight is that of the allosaur charging with its greater speed and leaping at the side of the Tyrannosaur, thus more than likely taking both of the beasts to the ground. But the more agile allosaurus would be first to get to its feet, if it even fell in the first place, and tear the larger dinosaur apart. (This is where the weak arms of the Rex disable him.)
Anyways, GO ALLOSAURUS!