As a proof of concept I recently tried to virtualize OS X (Mountain Lion) - It is important to note that VMware is now licensed to do so and you can read more here.
The following is an overview of the steps I followed to achieve my goal in some cases it was trial an error as I am not a regular Mac user.
The Hardware As OS X requires Apple hardware to run you will have to find yourself a Mac that will install and run ESXi.