It's repprted that Microsoft's next generation Xbox dubbed "Xbox 720" will feature an AMD Fusion II CPU.
According to reports, "It looks like the next-gen XBox will be based on technology from AMD based on production at Globalfoundaries on their 28nm 'high-k gate first' process." Intel and nVidia angered Microsoft after the initial Xbox release, which's what drove them to use the IBM Xenon processor in the Xbox 360.
While it's not known why Microsoft is now moving to AMD, one can't forget all the troubles Microsoft had when trying to make the current Xbox 360 backwards compatible. Microsoft had to invest significant resources to make games backwards compatible to the new platform, something you think they would try to avoid this time around.