[citation][nom]gamerk316[/nom]Very much doubt it, due to the architecture differences. You can get away with this for XBLA titles simply because of how non-resources intensive they are, which allows CPU side emulation of the PPC instruction set. But for 360 titles? Simply not enough horsepower for emulation to work. (Hell, emulation the 400MHz PS2 CPU is a major challenge!)[/citation]
Old games, maybe not.
New games, sure.
The next Xbox may be using PC components, which if true, will allow for you to buy and play your game on Windows or Xbox for a single purchase; a major selling point. If the next Xbox is using the same architecture as a PC, expect all games to be made for both Windows and Xbox.
It would make sense since: Xbox/Windows Live Games merge, Windows 8 Metro(buy your games through Windows 8 game store for Xbox/PC).