David Berlind at ZDNet, has an interesting analysis on Microsoft's licensing policy for Virtual PC on Vista.
Customers with an EA/SA can obtain Vista Enterprise, which has the right to install a second instance of an OS. In the retail channel, you will only be able to obtain Vista Ultimate, which does not carry these rights. With Vista Ultimate, you have to license every VM separately, and for most corporates this probably means buying an additional retail licenses. Interstingly the right to run a 2nd instance extends to both physical installations (a multi boot between two OSs installed in separate partitions) and running in a VM. While I'm sure MS want you to use VPC as the virtulisation platform, you can use VMware. Now given that VMware Server is shortly to be free, that gives Vista Enterprise customers some intreiguing options.