No source, it's just guessing, 128bit OS kinda gives it away.
Having said that MS desktop OS have traditionally been on a 3 year cycle (with the exception of Vista). The 3 year cycle combined with the beginnings of Win8 leaks and taking into account the IE10 preview and the previous IE9 release cycle puts a Win 8 release first half of 2012(ish).
There will not be a mainstream 128-bit CPU for many years yet. Maybe a decade or two.