As you may aware, significant improvement had been done on the latest hypervisor from VMware vSphere 4.1.
1 of the key changes are the vCPU for each virtual machine with latest NUMA hyperthreading architecture.

I found a very details and great post talk about this topic, please read the full post from here.

This will significant improve the Consolidation ratio and performance over the virtual machine VS the previous version ESX 4.0.