There is a great how-to over at Virtually Ghetto on enabling Nested ESXi on 5.1
There is also a great program called NEE over at VMware Labs, which I am lucky enough to have access to because I recently attended a training class, right before VMworld.
Nested ESXi is the strange act of running an ESXi VM on ESXi. You can also do it with Workstation, I recommend using Autolab is you are going that route.