The big news of the week was indeed Bungie splitting off, but in this 1UP interview, it was confirmed that they are working on another game based in the Halo universe.
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.