The PS3 is clearly more powerful than the 360. (if the MGS4 trailer wasn't proof enough than your beyond help).
Anyway since the PS3 is 3 times more powerful than 360 would it be possible to make an emulator for 360 on the PS3?
I say this because the 360 is using a G5 variant processor. Mac OSX is based on the G5. so if the cell processor can run OSX and linux than it must be partly similar to the G5.
Also the 360's cpu has 3 cores running at 3.2 GHZ the PS3 has 7 cores running at the same speed.
If an emulator could be made it would probably be run off the memory stick like the PSP homebrew is. Sombody could write a plugin to use the PS3's blu-ray drive to run the 360 games. Also since microsoft is planning to release a USB version of the controller you could plug the 360 controller in to PS3 and play the games in the emulator.
Give me your thoughts. It's probably not possible but it's a cool concept.