They are supposed to port the source engine to 360. I dont remember where I read it, but I think it was in game informer but I could be wrong.
http://www.xbox.com/en-US/games/backwardcompatibilitygameslist.htm They are always working on making all your Xbox games compatible. I would have hoped that this would have been a top priority before they released the system. The more games it was backwards compatible with the more people would want to buy it, like the PS2 was. It could play all your PS1 games. I can only hope the PS3 will be able to play all PS2 games.