Intel moaned that no money was being made from the CPUs being sold to Microsoft so it was scrubbing the entire project off as a marketing and brand awareness exercise, and the Imperium fell out with Nvidia.
As a result the Imperium went with an IBM-designed triple-core Xenon CPU that was (funnily enough) manufactured by Globalfoundaries and is now a 45nm part.
The current consoles run on an R600 derivative from ATI called the Xenos with performance close to a Radeon HD 1900 but with DX10 features from the R600.
But according to Tweaktown, the next-gen Xbox will be based on technology from AMD, based on production at Globalfoundaries on its 28nm 'high-k gate first' process.
It should manage 1080p at 60fps locked, or more, with 3D capable at the same frame rate. So in other words it will be an AMD Fusion II.