The Next Web claims that sources close to Microsoft have said that we should expect the pubic Windows 8 beta sometime in late February 2012. Windows 8 is expected in 2H 2012, but an exact date for release is currently unknown.
But, knowing the release date of the public beta paints a better picture of when to expect the RTM copy of Windows 8. If we use Windows 7's release milestones, we should expect Windows 8 hit the released to manufacturers (RTM) release in June. If this happens, Microsoft could enjoy a very, very good 2H of 2012, by selling its new OS, pushing tablets powered with the OS and pre-built systems with Windows 8 baked in.
What is unknown about the Windows 8 beta is what exactly will be included in the beta itself. No feature set is known, but we should find some information closer to release. I'm incredibly excited myself.