Microsoft has released Beta 1 of Virtual PC 2007. You can apply for participation in the program here.
New Features (emphasis mine):
- 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's much more information on the Virtual PC Guy's WebLog