Here is an excellent whitepaper titled “Understanding Microsoft Licensing for a Virtual Desktop Infrastructure (VDI) Environment” that simply explains the licensing requirements without needing a PHD to sift through the information and Briefs posted on Microsoft’s Overview of Licensing for Virtualization web page. More specifically I’m referring to the Brief on Licensing Windows Vista for Use with Virtual Machine Technologies.
It was written by Rodney Horton of SKT Communication Solutions. Rodney’s contact details can be found at the end of the document.