Virtual Machines

Microsoft released a brief in Jan 2008 to handle this question. The answer is essentially;

 Win2K8 Standard Per Install
 1 VM
 Win2K8 Ent
 Per Server
 4 VMs
 Win2K8 Data  
 Per Processor

This is specific to ESX, and you can't use the physical license you get with the product. Also, the licenses are set per physical server, or per physical processor (not cores) so they stay on the ESX host as the virtual machines move around (though you can reassign licenses every 90 days per license or per hardware failure)

So if you are going to run more than 4 machines per processor, then DataCenter is a better value.

(Don't forget the CALs or Internet Connectors)