Since yesterday a new support policy for Microsoft Virtual Server is available.
This policy now outlines that most Window Server software (except the ones from this list) is now supported in a Microsoft Virtual Server environment.
Unfortunatelly Microsoft SharePoint Portal Server is on the list of NOT supported software…
Just to add, here is an additional article you should be aware of when using non-Microsoft Hardware vitualization software: http://www.support.microsoft.com/kb/897615