In some scenarios, Microsoft found that the CAL policy surrounding Hyper-V use made the solution more expensive than VMWare’s. Customers who virtualized Windows Server 2003 required 2008 CALs even though they had purchased 2003 CALs. One of the primary reasons to virtualize an operating system is so that IT departments can consolidate their infrastructure to save money. If Microsoft decided to continue charging for the privilege to virtualize, then the cost saving incentive would have been negated.
So What is the New Policy?
With the change in our licensing policy, Windows Server 2008 CALs are no longer required if you are using Windows Server 2008 solely as a virtualization host. The only exception to this is if you are running Windows Server 2008 virtual machines, which would require Windows Server 2008 CALs
If you have volume licensing, please refer to the for detailed information Volume Licensing Brief. The crux of the matter is that if you are using any services on the host Win2k8 box, then you need to purchase/upgrade to the 2008 CAL. If you are using Windows Server 2003 and below or Linux, then no new CALs are necessary (2003 CALs still required though for Win2k3 instance).