A rumor for now, but it looks like it will happen.
Original source for TT:
Microsoft chooses AMD Fusion II for XBox 720 | KitGuru
AMD Fusion II on next-generation XBox :: TweakTown USA EditionMicrosoft's next-generation XBox is obviously being worked on and while the externals (the look and packaging) might be nowhere near finalised, hardware is having to be done right now and locked in with various companies with AMD looking like Microsoft's BFF for the new XBox.
Original source for TT:
Microsoft chooses AMD Fusion II for XBox 720 | KitGuru