I have here two valuable resources about Virtualization and Licensing. Yeah I know it's not that easy to understand but with the first tool you will be able to calculate how many licenses your environment needs. The second resource is about how Windows 2003 R2 is licensed while using other Virtualization Technologies.
Windows Server Virtualization Calculator (Version 2).
The new calculator lets you interactively build virtual machines running multiple Microsoft server products to estimate the licenses and costs for Windows Server by edition (Standard, Enterprise and Datacenter) and many Microsoft server products. It starts with configuring your server based on virtualization technology, sockets and clustering requirements, and then builds the virtual machines.
Licensing Microsoft Windows Server 2003 R2 to Run with Virtualization Technologies.
This document covers VMware ESX Server, VMware VMotion, SWsoft Virtuozzo, and Microsoft System Center Virtualization Machine Manager. This document also Intentionally compares VMotion, System Center Virtual Machine Manager and Microsoft Host Clustering (AKA Quick Migration) to build awareness with VMware customers that we have similar management and “VM movement” capabilities.
“Microsoft details Windows licensing for 3rd party virtualization platforms” blog: http://www.virtualization.info/2007/06/microsoft-details-windows-licensing-for.html
Currently I am reading a book on Virtualization written by Ben Armstrong - Professional Microsoft Virtual Server 2005,
I recommend you to read it it's a valuable resource.