Don't expect Windows 8 before October. That's the (perhaps unintentional) subtext of a recent Toshiba presentation captured by the folks at NetbookItalia.
In the presentation, Toshiba shows a cloud of "Portfolio Highlights you must not miss." There's "Two new Ultrabooks" in May, a "New detachable Performance Slate" (à la Eee Pad Transformer) in September, and... a little rectangle that says "Q4 2012" at the top and "Win8 Launch" at the bottom, with a Windows logo and a Metro screenshot sandwiched in between.
Unless it's a total guesstimate from a Toshiba PR person, this might be as close to an official launch time frame as we've gotten so far.
I wouldn't expect Windows 8 too late in the year, since Microsoft will no doubt want to capitalize on holiday sales. A release in October or early November sounds the most plausible—and the most consistent with history. Windows XP first hit stores on October 25, 2001, and Windows 7 did the same on October 22, 2009.