It seems that the backwards compatibility for Xbox 360 is only kind of sort of backwards compatible. How lame. Of course, the only games 99.999999% of people play are the not-so-great Halo and Halo 2, and I'm sure those will both work just fine.
http://www.gamesarefun.com/news.php?newsid=5103News - Clarification on Xbox 360 Backwards Compatability Provided
Posted by Ashton Liu at 02:28:55 AM EST on 6.7.2005.
Microsoft's company management has recently stated that the Xbox 360 would be backwards compatible with "most top-selling Xbox games." They also stated that Microsoft would support the Xbox well into 2006. However, since the current Xbox uses an Nvidia chipset as a graphics processor, as opposed to the new ATI chipset, some have expressed concerns about the backwards compatibility of the Xbox 360. Recently, Microsoft has explained and clarified the current situation.
Microsoft has actually agreed to pay Nvidia a small royalty in order to allow the ATI chipset on the Xbox 360 to emulate the Nvidia chipset's performance in reading certain Xbox games. The term 'certain' is used because currently only single-layered games will be emulated well with this hardware. For multi-layered games, there requires a further emulation in the form of 'patches' that Microsoft intends to include with the hard drive packaged along with the Xbox 360. One can assume that most Xbox games will be playable by the Xbox 360 in this way, but not all. However, Microsoft has promised that further patches will be added in case customer demand warrants the action.