http://blogs.zdnet.com/microsoft/?p=2621

MJF: Will Virtual PC 7 be available independently of Virtual XP? Or are the two only available hand-in-hand?

Woodgate: Customers can use Windows Virtual PC independently of Windows XP Mode. We expect developers and IT Professionals to do this, as they do today, and we catered to these folks by adding the commonly requested support for USB to Windows Virtual PC.

7 years too late.....