Our VMware vSphere 4 Enterprise license covers us for for 2 physical CPUs (1-6 cores per CPU), we are running it on a 2 CPU x 4 core box giving us 8 virtual CPU’s but as we discovered we can only allocate 4 virtual CPU’s per host.
TO resolve this we would have to purchase an Enterprise Plus license per cpu for a 2 cpu server. In fact to enable failover and VM ware high availability features we would also need to purchase a set of licences for an additional server.
We are currently evaluating the best way to provide a high availability SQL Server environment. At the moment we are using MSCS we felt we may be able to better utilise server resources if we could move SQL Server to VMware but this may not be justifiable given the license costs of Enterprise Plus
VMware have a KB about it here http://www.vmware.com/products/vsphere/buy/editions_comparison.html
VMware also have a Compare vSphere Editions chart at