On Bungie's latest podcast, the team - between explicit statements that it's far too early to be talking about their next project - go ahead and talk about their next project, and aren't stingy about speaking in the third-person, either:

"Bungie's next game - that isn't Halo 3 expansion, DLC-type stuff - will be something totally different from Bungie."

Not terribly insightful read in isolation, but it makes more sense when you consider it comes on the tail-end of a discussion about how the Wii and Rock Band have shown that a lot of gamers want "happy games", not just to "pwn or be pwned". Happy, huh? A platformer with fluffy clouds, kids in puppy suits and fully-integrated co-op would be unexpected. Unexpected and awesome.
Source:http://kotaku.com/353160/bungie-talk...-its-different

Wow! I can't wait!