As published on VMwareInfo.com:

So there were a bunch of unconfirmed plans last year (2009!) that VMware might be getting rid of the Enterprise edition of vSphere in favor of the more expensive feature rich Enterprise Plus.  I think there was a lot of push back from customers and VMware issued a stay of execution for Enterprise licensing.  Sort of.  Looks like the current stance is that EXISTING customers with up to date subscriptions for VMware 3.5 Enterprise licenses can renew/upgrade to VI4 Enterprise licenses but NEW customers or NEW licenses only have the Enterprise Plus option.  Enterprise without plus is not an option for new licenses.  If you are confused, then this chart probably won’t help much.

As posted on VMwareInfo.com:

Here is a quick one page Visual Guide to help discern the differences between the various licensing models of VMware’s vSphere.  For one, they have switched licensing back to SINGLE processor licenses and have scrapped the FlexLM license database and went to easier 25 digit license codes.  The four editions of vSphere 4 will be Standard, Advanced, Enterprise and Enterprise Plus.