Via Sam Stephenson, I learn about Vista’s licensing terms:
Parallels is an indispensable tool that lets us test our applications in all the browsers we support on a single computer. We can even test IE 6 and 7 side-by-side by running Windows XP in one VM and Vista in another. As a web developer, Parallels is easily the single best reason to own an Intel Mac. But starting with Vista’s release, Microsoft wants us to pay for the privilege: you’ll legally only be able to install the most expensive versions (Ultimate and Business) in a VM, even if all you’re doing is clicking on things in IE.