Microsoft has made some changes to licensing with Windows 2003 R2 Enterprise Edition. If you purchase Windows 2003 R2 Enterprise Edition, you can also run four virtual instances of the server software inside Virtual Server 2005 R2 for no additional cost. Also, it’s four running instances; hypothetically you could have 50 virtual instances but you can only have four running at any one time unless you want to incur additional licensing costs.
The exact verbiage about the licensing changes is:
“Windows Server 2003 R2 Enterprise Edition – Each software license allows you to run, at any one time, one instance of the server software in a physical OS environment and up to four instances of the server software in virtual OS environments on a particular server.”
This is a change from the current licensing scheme, in which you pay for every installation whether it's running or not. This is Microsoft’s move toward a more virtualized environment.
You can find more information in the Microsoft Virtualization_brief.doc document