It sounds like the same deal Nintendo and Rare came to during the middle of the N64 era. Basically if the relationship sours, Bungie can move on and only have to give up Halo. Any new titles published by MS from here on out will be Bungie's property. Kind of like how Nintendo published Killer Instinct, Conker, Jet Force Gemini, and Banjo Kazooie but the IPs actually belonged to Rare.
The deal can work out well for both parties. If Bungie can't produce any addition bankable hits for MS, then MS can sell their shares in Bungie to another company like Nintendo did with Rare. If Bungie keeps cranking out big hits, they can use that leverage to get more of a cut in the profits from MS or choose to just buy out MS and become completely independent.
EDIT: As for as exclusivity goes, most likely their titles will continue to be exclusive to th 360 for the forseeable future. If Bungie and MS split though they'd be free to remake those titles on other plaforms. I hate to keep bring up the Nintendos deal with Rare but this deal seems indentical to it.
