Well, they were the first (as I can recall) to show how you could virtualize ESX 3 on VMware Workstation 6 – and the boys at xtravirt.com have done it again with ESX 4.
The PDF is here… http://xtravirt.com/user?destination=/node/177
BUT, unfortunately, you do have to register with the site to get the PDF file.
If you want to virtualize ESX4 on ESX4 you will need processors that support Virtualization Hardware Assist (Intel-V or AMD-V) and make sure the VM that runs the virtualized ESX host is patched to a vSwitch/Port group that has been enabled for promiscuous mode. But after that it is very easy to just install ESX into a VM.
No fancy edits need to be made to the VM to make this happen – but if you then want to create a VM on the virtualized ESX host you will need to modify the .VMX file to allow that to happen:
1. Right-click the virtualized ESX VM
2. Select Edit Settings
3. Choose the Options Tab and Select the Configuration Parameters button
4. Click the Add Row button
5. In the name column add the monitor_control.restrict_backdoor entry
6. In the value column set it to be true