I found brief announcement about this on http://blogs.msdn.com/iainmcdonald/archive/2008/02/04/whoo-windows-server-2008-is-released.aspx.
Today Microsoft officialy announced that Windows Server 2008 and Windows Vista SP1 are RTM'd. Here are posts on WIndows Server Division weblog http://blogs.technet.com/windowsserver/archive/2008/02/04/windows-server-2008-rtm.aspx and on Windows Vista Team Blog http://windowsvistablog.com/blogs/windowsvista/archive/2008/02/04/announcing-the-rtm-of-windows-vista-sp1.aspx. Here is an official MS PressPass document http://www.microsoft.com/presspass/features/2008/feb08/02-04WS2008.mspx on Upgrade Options and Ensuring Compatibility.