since some time ago it's possible to run esx(i) inside a vmware Workstation. In others words, running the esx(i) host as a virtual machine.
To be able to run esx(i) virtual, the host where Workstation is running needs to accomplish some requirements because esx(i) runs as a 64 bit virtual machine.
If you use the Workstation 7.X or later, running esx(i) virtual is a lot of easy. Just select the "Vmware ESX" like the guest operating system when creating the virtual machine. Anything else is needed.
But if you use a older version than 7.X, some adjustments need to be done. Check this guides to learn how to do it:
Some interesting notes from vmware about running ESX(i) virtual.
Here more information about this topic.