After weeks of seemingly unbelievable rumors that a company would actually be able to impose its will on Microsoft, it has happened.  Bungie is no longer under wholly owned by Microsoft.  Bungie is now a privately held company, Bungie LLC, and will be able to "expand both creatively and organizationally," as said by Harold Ryan, studio head of Bungie. 

There's no word yet on where Bungie will go from here.  Some rumors flying around include a re-make or porting of Marathon, a classic Bungie FPS as well as the possibility that Halo 4 is already in development (though both of these are unlikely).  Regardless, this is a huge loss for Microsoft if Bungie stops with exclusivity on the Xbox 360. 

 Source: http://www.mygamer.com