Microsoft is aiming at an April 2012 date for the RTM release of not only Windows 8 for x86, but for Windows 8 for ARM/SoC and Windows 8 Server all at the same time. Microsoft's plan is to deliver a single Windows 8 beta build around the the time of its upcoming Build conference.
Microsoft hope to convince Windows, Windows Azure, Windows Phone and Web developers to attend the event in Anaheim, California between September 13 and 16, 2011. Once the beta hits, Microsoft will then deliver an RC (release candidate) build of Windows 8 which should arrive in January 2012. The final released build will be the RTM, this means only two public build tests which is the same method of what we had for Windows 7.