This is a common question I get from customers...
Q: Does Microsoft support running WSS v3 and MOSS in a virtualized software environment?
A: Unlike SPS2003, there will be product support for WSS v3 and MOSS on Virtual Server 2005 R2 x86 or x64. Good news. Oh, take note, no product support for Virtual PC 2004.
You might want to check out this support policy.