Virtualization rights for Windows can be accomplished through Software Assurance for Windows (which gets you Windows 7 Enterprise Edition that includes virtualization rights), or through the Windows Virtual Access license through Volume Licensing. We have a PDF available that discusses Windows virtualization benefits in Software Assurance here: http://download.microsoft.com/download/5/c/7/5c727885-ec15-4920-818b-4d140ec6c38a/SA_Customer_Virtual_Desktop_Brochure.pdf and we have a page on Windows virtualization licensing here: http://www.microsoft.com/licensing/software-assurance/windows-virtualization.aspx. In addition, we have an all-up Windows (including Server) Virtualization licensing page here: http://www.microsoft.com/licensing/about-licensing/virtualization.aspx.
Ever wondered about Virtualization Right for Windows??