I do not think you can run ESX in a VM running on VMware Server - at least I have never heard of anyone doing that - it is possible to do it under VMware Workstation - the reasone why is the difference of how Workstation and VMware Server do their virtualization -
This is only possible within Workstation and not Vmware Server. Check out http://www.xtravirt.com for help on getting ESX v3 running within Workstation.
Edward L. Haletky
VMware Communities User Moderator
Author of the book 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', Copyright 2008 Pearson Education.
CIO Virtualization Blog: http://www.cio.com/blog/index/topic/168354
As well as the Virtualization Wiki at http://www.astroarch.com/wiki/index.php/Virtualization