This dude understands making fun games, he's one of the reasons I can't wait to play Second Son. I'd like to see him go to 343 and bring back that old-school Halo gameplay he was a major part in making so fun with the first three games.
As stated in the Marc Whitten thread these things happen after a game ships.
I think there is a chance he is joining another studio based in Seattle. If he was going indie, he would likely have mentioned it. By staying silent on future plans there is likely a Press Release that he can't comment on.