At the World Partner Conference, Microsoft has provided details about the release dates for Windows 8 and Windows Server 2012.
More details about Windows 8 here: http://windowsteamblog.com/windows/b/bloggingwindows/archive/2012/07/09/upcoming-windows-milestones-shared-with-partners-at-wpc.aspx
Windows 8 RTM, first week of August with general availability in late October.
More details about Windows 2012 here: http://blogs.technet.com/b/windowsserver/archive/2012/07/09/windows-server-2012-final-release-timing.aspx
Windows 2012 RTM, first week of August and generally available to customers through multiple channels in September.