Virtualisation Support Wizard now available
Microsoft's Virtualisation Support Policy Team has been working for months on development and testing so that they can clearly define, not surprisingly, virtualisation support policies. The result is the Virtualisation Support Wizard which you can use to see if your virtualisation configuration is supported without needing to call the great people at Microsoft support.
Virtualisation Troubleshooting and Support TechCenter on TechNet
The observant will notice that BizTalk Server 2009 isn't on the list yet - I've raised this omission as the product was finalised back in April.
[[Edited July 2nd 2009]]
The Virtualisation guys have told me that the product was missing because BizTalk Server 2009 is not referenced in:
957006 Microsoft server software and supported virtualization environments
so I've submitted a Change Request to get the KnowledgeBase article updated too.
The following article, though, is already up-to-date:
842301 Microsoft BizTalk Server supportability on a virtual machine