Yes, that's what I've been thinking too.Well, the PS3 is pretty consistent with its powerdraw (either Idle,XMB or ingame). I guess when playing PS2 Games you dont tax the Cell or RSX much, but you have the PS2 Hardware switched on and drawing additional power.
I don't have disc-based PS3 games yet, just the Gran Turismo and Ridge Racer demos. A disc spinning in the drive will probably add a couple more watts, and who knows, maybe there are ways to stress the machine harder. These two racing games seem to excercise mostly the graphics chip, if anything. I could try Lemmings