A holographic sentient AI sent word that Xbox.com has the skinny on how backwards compatible the 360 is:
Todd: Yes. Today we’re confirming more than 200 titles including games like Halo® and Halo® 2; Star Wars® Knights of the Old Republic® I and II (LucasArts); Ninja Gaiden® and Ninja Gaiden® Black (Tecmo®); and Grand Theft Auto III, Grand Theft Auto: Vice City, and Grand Theft Auto: San Andreas™ (Rockstar Games). I can confidently say our goal remains to make as many games backward compatible as possible.
Xbox.com: So what are the 200+ games that are backward compatible as of today?
Todd: It's simple: You can log onto Xbox.com/games for an up-to-date list of original Xbox games that run on Xbox 360. There are more than 200 titles as of today, and we will update you on a regular basis after launch.
More links and info from the links above as well as a list here. Recent mumblevine couldn't peg this down, so it's good to see that Microsoft is taking the effort seriously.