As blogged about on the Windows Server blog, what was previously referred to as Windows Server virtualization or codename "Viridian" has now been officially named Microsoft Hyper-V Server.  The aforementioned blog posting also lists the various Windows Server 2008 SKUs and some minor licensing changes with respect to virtualization that you should take a look at.  Note: Hyper-V Server is still slated for 180 days after the release of Windows Server 2008.