FYI - I use this for all my testing, including my WSS v3 on SBS 2003!! 

Virtual PC 2007 Beta now available!

Good news - the Virtual PC 2007 Beta is now available.  You can sign up for it here: https://connect.microsoft.com/programdetails.aspx?ProgramDetailsID=874.

Major changes include:

• Support for hardware virtualization (Intel and AMD)
• Support for Windows Vista as a host operating system
• Support for Windows Vista as a guest operating system
  • Note: No support for Aero Glass though - just Aero Basic
• Support for 64-bit host operating systems
  • Note: Virtual machines are 32-bit only
• Improved performance

There are also a plethora of bug fixes.  Some notable ones include:

• Lots of work to allow Virtual PC to play better with laptop power management
• Fix for IntelPPM issue (http://blogs.msdn.com/virtual_pc_guy/archive/2005/10/24/484461.aspx)
• Virtual PC now supports greater than 2.2GB ISO images

Source: Virtual PC Guy's WebLog : Virtual PC 2007 Beta now available!