In VMware Workstation 6.5 beta 1, it is no longer possible to run ESX in a VM. This is because of some improvements that  have been made in the code that unfortunately also prevents ESX from running in a VM.

I’ve been using this possibility in Workstation quite a lot for product testing and product demo’s and would love to get it back. I would even settle for less options in such a VM or for less performance. As long as its not gonna take an hour to start the VM, like it does without VT enabled.

See this thread: http://communities.vmware.com/thread/117676?tstart=0

If you would also like to be able to run ESX in Workstation, post into this thread to make VMware aware how often you use this functionality.

by Gabe