My guess is Xbox360 would have trouble emulating Xbox games because the systems are not that much different. Now before people kill me, what I mean is the technology is rather close to each other compared to say... teh Revolution and the NES.
The Revolution can emulate NES/SNES/etc just as a PC today can. The X360 is (for this argument) a very high end PC.... and they cannot currently emulate Xbox games.
With the PS3, they are NOT emulating the games.... you are using the original media. I know that the PS2 had many, if not all, of the processors from the PS1 built it... so it was no problem for the PS2 to act like a PS1. I'm not sure of the specifics of PS3, but I would say what makes them have "no problems" with PS2 games is the fact that there will be some similar CPUs or processes in the system... either that or the Cell truly is powerful enough to emulate the PS2 hardware.
Either way, for my money, it seems that only PS3 and Nintendo (at least with Gamecube)is truly backwards compatible. Everything else is a totally different ballgame.