There has been a whole lot of discussion about the possibility of Bungie leaving Microsoft. Some of the feedback from the community has been "they can't leave Microsoft owns them" or "why would Microsoft let them leave".
This is only my opinion and not based on any knowledge specific situation.
The idea of Bungie breaking away is plausible because Microsoft owns Halo, not Bungie. The company was acquired in 2000 by Microsoft to ensure the development of quality games to launch a little system called the Xbox.
If, and this is a big if, the developers at Bungie could buy their way out of their agreement with Microsoft then they would be free to develop games for other consoles.
I am sure they would come back to do Halo 4 , but I would bet that Sony would love to have them develop an A-list exclusive for them as well. Looking at the success of companies like UbiSoft and Epic has to have the folks at Bungie wondering what are they leaving on the table as far as dollars go being tied to Microsoft. This could also be a win in the short term for Microsoft because they may retain the distribution and publishing rights to select future titles for a timed period after the split.