1/29/10: Thanks! Windows SBS 2008 is now listed in the subject KB as a supported guest operating system on a Hyper-V Server.
***Microsoft's list of guest operating systems for Hyper-V / Hyper-V Server don't mention support for SBS 2008 (although they list almost every other flavour). I'm guessing that it's just an oversight, given this extensive TechNet section.
http://support.microsoft.com/kb/954958