Here is the link to the Microsoft document on Datacenter server licensing.

http://www.microsoft.com/windowsserver2003/evaluation/news/bulletins/datacenterhighavail.mspx

This is the relevant portion:

Unlimited virtualization rights: Starting October 1, 2006, new servers licensed with Windows Server Datacenter Edition (and previous licenses with new version rights) will have license rights to run an unlimited number of virtualized Windows Server instances. By simply licensing the server's processors with Windows Server Datacenter Edition, customers will be able to run Windows Server Standard Edition, Enterprise Edition, Datacenter Edition or a mix of the three editions without having to track the number of virtual machines or pay for additional Windows Server licenses.

Licensing does not depend on which virtualization technology is used. With all processors in a server licensed for Windows Server 2003 R2, Datacenter Edition, you can run one instance of the software in a physical operating system environment and an unlimited number of instances in virtual operating system environments. With VMWare GSX Server or SWsoft Virtuozzo, this means you can run one physical instance plus unlimited virtual instances. With VMWare ESX Server, it means you can run unlimited virtual instances because there is no need for a physical instance.