According to an article on Eurogamer it is..

While internet news sources today have reported that Microsoft is readying to launch its next generation Xbox hardware in America and Europe in the fourth quarter of this year, Eurogamer has learnt that the specific timing for the machine's roll-out is late October or early November, and merely weeks after the US launch. The exact date is yet to be solidified.

Thing is, are Micro$oft panicking that the PS3 will gobble up all before it? I know the XENON will have to pull a rabbit out of a hat to claw back Playstation that's for sure