I have been having fun the past couple of days installing ESX 3.02 under vmWare Workstation 6.

I won't bore you with all the technical details, however there is an excellent write-up here. 

I have since ran into a snag when trying to install XP under my virtualized copy of ESX.  During the XP setup, XP would complain that it could not find my HD.  I ran across this wondering article which worked flawlessly.  I did, however, use the BusLogic driver.