Microsoft is changing things up a bit as they launch Windows 2012, the new server OS. They are going to just two editions:
Standard Edition and Datacenter Edition.
Any copies of Enterprise Edition that are owned in the world (in Software Assurance) will convert to 2 standard licenses.
All features will be avaliable in both editions of Windows 2012. The only difference being the virualization allowance. With each license of Standard that you apply to a server you will be able to host up to two VMs. You can stack copies of Standard on a server to increase that number as well.
Datacenter Edition will continue to provide unlimited free VMs.
The licensing for both editions is based on a two processor server and you license servers by their physical hardware (in other words VMs cannot be licensed).
With the move to 2012, the Enterprise and Web Server versions are being elimiated. One to reduce the costs / complexity of licensing and one due to the lack of popularity.
You can read more here: http://download.microsoft.com/download/4/D/B/4DB352D1-C610-466A-9AAF-EEF4F4CFFF27/WS2012_Licensing-Pricing_FAQ.pdf
