I stumbled across this download today on Microsoft's site, but the KB article 948515 doesn't exist yet and even the virtualization team blog doesn't mention it! Hey ho, I'm sure they'll blog it soon! So it seems that they have releaed today an update for Virtual Server 2005 R2 SP1 that provides support for the following Host and Guest Operating Systems
Additonal Guest Operating System support:
Windows Vista® Ultimate Edition with Service Pack 1 (SP1)
Windows Vista® Business Edition with Service Pack 1 (SP1)
Windows Vista® Enterprise Edition with Service Pack 1 (SP1)
Windows Server® 2008 Core
Windows Server® 2008 Standard
Windows Server® 2008 Datacenter
Windows Server® 2008 Enterprise
Windows Server® 2008 Small Business Server
Windows XP Professional with Service Pack 3
Additional Host Operating System support:
Windows Vista® Ultimate Edition with Service Pack 1 (SP1) (non-production use only)
Windows Vista® Business Edition with Service Pack 1 (SP1) (non-production use only)
Windows Vista® Enterprise Edition with Service Pack 1 (SP1)(non-production use only)
Windows Server® 2008 Core
Windows Server® 2008 Standard
Windows Server® 2008 Datacenter
Windows Server® 2008 Enterprise
Windows Server® 2008 Small Business Server
Windows XP Professional with Service Pack 3 (non-production use only)
I haven't tried it on my Virtual Server yet, but will do soon and let you know what i find.
You can find the update here: http://www.microsoft.com/downloads/details.aspx?FamilyID=A79BCF9B-59F7-480B-A4B8-FB56F42E3348&displaylang=en