Halo has been quite the cash cow for Microsoft and Bungie, but even it must come to a (relative) end - according to Bungie, the upcoming prequel Halo: Reach will be the last title they develop for the "current Halo trilogy."
Speaking with VG247, Bungie's community manager Brian Jarrard said that Halo: Reach will be the last game the studio will develop about the Master Chief and humanity's war against the Covenant and Flood.
For now Reach is definitely our next big thing, and it doesn't really fit into the Halo trilogy itself. It's definitely a prequel, standalone offering, so we don't have current plans to go back and inject more content into the current Halo trilogy ... I think ODST is going to be a nice exclamation point, a nice rounding up of multiplayer content and a different side-story, but right now Reach is our focus.
Of course, one has to note the phrase "current Halo trilogy," which seems like a sneaky get-out-of-jail-free card to me. What's to stop them from doing another game and calling it a sequel, eh?
Reach will come with a multiplayer beta when ODST ships September 22nd and is slated for a release in Fall 2010.