Following earlier indications that the Xbox 360 will only be backwards compatible with some Xbox games, Microsoft has admitted that existing software will need to be recompiled before it can be run on the new console. The problem, it says, is down to hardware incompatibility - since the current Xbox uses an Intel processor, but the 360 will use IBM's PowerPC architecture, while NVIDIA's graphics solution is being replaced with an ATI one.
News source: GamesIndustry.biz