I can think of two (obviously highly speculative) possibilities that seem to fit the vagueness about compatibility:
1) Upcoming high-profile Xbox releases will be transitional dual-system discs, containing executables for both systems with shared data. This seems consistent with a lot of the XNA hype and might help ease the transition from Xbox to Xbox 360 development for mid-size and smaller development teams.
2) Have developers port Xbox bestsellers (both past and upcoming) to 360 and the executables can be downloaded through Live, but will use the Xbox disc for data, or they could have an emulation framework with game-specific tweaks/hacks and QA instead of full-blown ports. Either way the result visible to the player would be roughly the same.
But wouldn't you think that would add more pressure and cost on the devs and pubs to "re-issue" there old/current games on the new system. They could spend that extra money developing new games for the 360