Good news: ISA and Forefront Threat Management Gateway are now supported on hardware virtualization platforms validated by the Microsoft Server Virtualization Validation Program (SVVP),
A white paper available here provides specific guidelines for deploying ISA and Forefront TMG within hardware virtualization. Recommended reading - particularly the best practices documents provided in the References section.
And below is a TechNet Edge video with Jim Harris providing some guidance on what to consider when virtualizing ISA and TMG woth Hyper-V or Virtual Server.
Virtualize your ISA or Forefront TMG servers