If you go to the 'about' for Windows Server 2008, you'll see the box to the right, showing Windows Server 2008 as 'Version 6.0 Service Pack 1". I'll admit to never having noticed this before ...
But, Iain McDonald (in Windows Server engineering) did an amazing blog on the history of the client OS's and the server OS's -- written by the man who knows it and is definitely worth reading.
Again, this is not my area of expertise -- but as an MCSE since Windows NT 3.51, and having watched Windows NT4 and 95/98, 2000 server and workstation, XP and Windows Server 2003, and now the latest Vista and 2008 generation. It really provided some enjoyable and understandable history.
Thanks Iain !