In what has been a rock solid front from the two companies over the years, a fracture has appeared in the form of Bungie being pushed aside from the Microsoft press conference due to "time issues".
To add fuel to the fire, head writer of the Halo series, Frank O'Connor, had left Bungie to go and "work more closely with Microsoft on Halo" this May, suggesting that more Halo titles could be in development as genuine first party titles. And with MGS owning the intellectual property for Halo, it's probable that we will see a similar situation here as with Crytek and Ubisoft over Far Cry 2.