The Virtual PC 2007 Beta is now available at connect.microsoft.com

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
Virtual PC now supports greater than 2.2GB ISO images

Via Virtual PC Blog