I think going by the initial rumor and the press release, it's pretty obvious what's going on here. Microsoft wants to make Halo Kart and Halo Party and Halo 4: The Haloening and more Halo games forever! Bungie, so desperate to move on to something else, and finally big enough to tackle multiple creative projects at once, bought themselves out so they could do so. In the process, they give up the golden egg to Microsoft and the software juggernaut also has first right of refusal for an undisclosed period of time.
After that, they will flee to PlayStation 3!