Microsoft has released the compatibility list for original Xbox games on the Xbox 360.
The 212 titles will be run though an emulator like software that will have to be installed on the hard drive of the 360. That means those without the hard drive are going to be out of luck when it comes to their older titles. Also it is unknown how the emulation software will get on the Xbox 360. Does it come with it, downloaded via Xbox live, or seperate download and installation?
The emulator software will be upgradeable as time progresses, meaning the 212 titles listed, will not be the only titles to be backwards compatible. It is sad that they could not have done a better job on making the Xbox games work 100% on the Xbox 360. I know some people that have hundreds of dollars of Xbox games and they probably will cringe at the thought that they have to run through an emulator, and might not work.