In an effort to once and for all clear the air with the whole "Windows 7 Server was always planned to be Windows Server 2008 R2," Stephen took up a timeline of screenshots in chronological order from presentations found right on Microsoft's servers. This basically shows the shift in timing where Windows 7 Server went from being planned as a major release to an update release (or R2, if you will).
What Was: Windows Server 2008 (Codename Longhorn Server) -> Windows Server 2008 R2 (Codename Longhorn Server R2) -> Codename Windows Blackcomb Server (Blackcomb became Vienna and Vienna became 7, so it was thought Windows 7 Server would indeed be what was to be Windows Blackcomb Server)
What Is: Windows Server 2008 (Codename Longhorn Server) -> Windows Server 2008 R2 (Codename: Windows 7 Server) -> Codename Windows 8 Server
2004 - 2005 (Windows Blackcomb Server): In presentations related to Windows Server 2003 and Windows Server 2003 R2, these roadmaps had Blackcomb Server coming after Longhorn R2. (Codename Longhorn Server became Windows Server 2008, so, Longhorn R2 would have been Windows Server 2008 R2 by these roadmaps).