In a really stupid way this would make sense for Microsoft and Bungie.  Bungie has already moved off of the Microsoft campus and Bungie has said they are done making Halo games.  Microsoft owns 100% of Bungie and owns all rights to the Halo franchise.  Microsoft has been closing down the development studios and absorbing the teams into Microsoft proper.  This very well could be happening to Bungie now that Halo 3 has been released.  Who really knows what's going on.  The fact that Halo 4 is already rumored to be in development means it could be done by a different developer.  Really who knows if this is true or not but I would have expected Micrososft's PR to have swatted this own by now.

http://www.joystiq.com/2007/10/02/rumorbusters-bungie-is-leaving-microsoft/