The first Service Pack (sp1) for Microsoft Virtual PC is now available for download.  I'm not sure what all of the changes in this service pack entail, I couldn't find the info at the time of publishing.  I did however note that the following additional host and guest operation systems are supported:

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 Standard
• Windows XP Professional with Service Pack 3

Additional Host 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 XP Professional with Service Pack 3

Happy virtualizing!