Though Bungie's Lars Bakken doesn't go into tons of detail about the studio's next game, he does say, "We're already working on a new IP that we can't talk about yet ... we've done a lot of different styles - first person to third person to RTS - so it could be anything." Eurogamer found out earlier in the day from Bakken that the upcoming game has yet to find a publisher, though Bungie is currently in talks with various companies and he says, "those talks are, I guess, close." Future Halo titles (you thought Reach was the last?) will be handled by 343 Industries, the recently trademarked, Microsoft-owned studio.
We contacted Bungie for comment on Mr. Bakken's comments and were told, "While it's great to see so much public interest over what the future holds for Bungie, we're not ready to talk about any plans or games beyond Halo 3: ODST or Halo: Reach. It's no secret that we've had multiple projects in development nor that we've also been working on original IP in tandem to our Halo titles but our focus right now is on our two upcoming releases. We'll have more to say when the time is right."
Source 1 - IGN UK
Source 2 - Eurogamer