KB 897614 , Windows Server System software not supported within a Microsoft Virtual Server environment, was updated on November 30. The line that interested me the most was: ISA Server 2006 is currently supported running within Microsoft Virtual Server 2005 R2. A snippet of the KB is below:

The following Windows Server System software is not supported within a Microsoft Virtual Server environment:

Speech Server requires telephony hardware not available in a virtualized hardware environment. Therefore, Speech Server is not supported when run within Virtual Server.

ISA Server 2000 is currently not supported running within Virtual Server.

ISA Server 2004 is currently not supported running within Virtual Server.

ISA Server 2006 is currently supported running within Microsoft Virtual Server 2005 R2.

SharePoint Portal Server is currently not supported running within Virtual Server. Support for SharePoint Portal Server within Virtual Server is expected in a future release.

Identity Integration Server 2003 is currently not supported running within Virtual Server.

The Identity Integration Feature Pack is currently not supported running within Virtual Server.

According the the Network Load Balancing (NLB) support team, NLB is not supported within a Virtual Server image. Full Story At Source