Windows 8 will likely release in the summer of 2012, but we could see the first beta version of it arrive September 2011 thanks to a road-map that has leaked. Although the specific date is not mentioned on the road map, what we do see is that the M3 coding is about to begin.
This means that the M2 milestone phase is about finished and when M3 is complete a beta could roll out, followed a few months later, perhaps at CES, with another beta and then on to the final product around the summer of 2012.