A slim version of the 360 will undoubtedly come - probably in time with the Natal release.
My guess is next holiday.
Project Natal alone or packed with new slim 360, Halo: Reach, possibly Gears3, possibly Fable III and who knows what launch Natal titles.
Edit: for clarification, I'm not suggesting that any of those titles will be bundled with a slim natal 360, just that that's what will be available for next holiday.







