In an interview with gaming site 1UP Bungie's Community Lead Brian Jarrard revealed that Halo 3 won't be the last Halo game from Bungie.
The firm is already working on a new Halo game, it won't be "Halo 4" but it will be situated in the Halo universe.
1UP: Is Bungie now contractually obliged to give Microsoft first-look on future titles, Halo-related or otherwise?
Jarrard: We aren't at liberty to discuss the specifics of this business arrangement but suffice it to say that we look forward to continuing to work with Microsoft as a great partner and world class publisher.
1UP: For all the super fans: does this mean Bungie won't work on whatever "Halo 4" turns out to be?
Jarrard: Well we are already at work on another Halo game -- I wouldn't call it "Halo 4", but it is based in the Halo Universe and represents a collaboration between Bungie and Peter Jackson. Halo 3 was definitely not the last Bungie Halo game.