That depends on what type of clients you are using.
If you have Windows clients with SA (software assurance) then you don't need anything extra to use VDI. If you are using thin clients or OEM licensed desktops, you need Windows VDA (Windows Virtual Desktop access). Windows VDA is a device-based subscription license and will be available at $100/device/year.
http://www.microsoft.com/windows/enterprise/solutions/virtualization/operating-system/
Sven Huisman
VMware vExpert 2009/2010
