I see a small problem, different hardware.
See that is the idea about consoles the game developers have access to game kits that help them make games that run perfect on said set hardware. Unlike a PC consoles only have one platform with a known set of hardware functions. So no matter if you have an old PS3 or a newer one they each can play the same games and each react the same to the games code.
The Xbox 360 is a whole different platform with different architecture then the PS3. Even if someone wanted Halo 3 is written for the Xbox 360, porting it to run on PS3 hardware would be a long process indeed.
Take for instance the processors already the xbox 360 has 3 cores, the ps3 has one core for general usage and 7 SPU's, streaming processors and those need to be run very different.
Those who differences already make it though to make games for multiple platforms work equally well. Let stand running a game optimized for one platform on another.
Just not possible...Unless one of the two is about 200x more powerful in such case you can run an emulator.