cjrgreen: When they did the Xbox 360 GPU, they also went with an AMD chip, and it was similar to the then-current 24xx series, but they added features to it like the eDRAM that made it very different to program. I'd expect something similar this time; they will start with GCN but get AMD to add something their developers really want.
wodmarach: Other way round it was the test chip for the 2400 series shaders even though 2400 hit consumers earlier it taped out later
According to AMD, the development of Xenos involved a large separate team. They already had the unified shader design in place before either one, but you're right; the first use was in Xenos, and the 2xxx were later. Considering the whole 2xxx series fell into the "that dog don't hunt" category, it is surprising that Microsoft was able to get so much out of Xenos for so long.
The most popular rumor I have been seeing is that the "720" will use a 6-core Bulldozer-like Fusion APU. And again, just because that's the way Microsoft swings, I'd expect it will have some bells and whistles that retail Bulldozers don't.