"The 360 will be, and has been stated numerous times, completely backwards compatible, but this will take years. At launch, it will be backwards compatible with only the top selling games, then updates to the emulator will be issued throughout the following months."
Has it? Seriously, we always see this said, but I don't think I've ever seen Microsoft say too much official either way. Even those Xbox 360 marketing flyers say top-selling games in a sort of lets hedge our bets, it could be all, it could be 3 games that are backward compatible kind of way.
Even with the BS Microsoft spews I doubt they even know for sure what is going on entirely. Though I doubt it'll take years. I figure if we don't see backwards compatibility fairly quick they'll ignore the rest of the games.