In an interview with Seattle Times Bungie studio head Harold Ryan made the following statement regarding the use of Project Natal with the upcoming Halo: Reach:
"I absolutely think 'Reach' could be enabled with it"
This is far from a confirmation of the use of Project Natal with Halo: Reach, but using Project Natal to some extent with the next big Halo title would make a lot of sense from Microsoft's perspective.