Reader Roger Zan has been corresponding with me recently on his attempts to get x64 virtual machines to boot inside an ESX x64-based VM-in-a-VM environment. So far he's been unsuccessful. He first wrote me that:

Even though VM Workstation 6.5 can fully host 64 bit VMs', there is an additional layer I am going through within the ESX VMs' that is somehow not passing the 64 bit CPU extensions through to the 64 bit guest OS.

This means that when running an ESX enviornment within a VMware Workstation environment, it is not possible to build an x64 virtual machine that is hosted by ESX. He digs a little deeper to find out why:

Basically, your Workstation will use the VT (or equivalent) hardware assist technology to be able to run the vmkernel. Now, from within the ESX, you will only be able to run BT (Binary Translation) virtualization, hence restricting you to 32 bits. That works because when doing so, you are using two different technologies to virtualize and they are "transparent" one with the other. If you try to run a 64 bit VM, the ESX, despite believing it's actually capable of doing so (because the processor advertises the option i guess), realizes it cannot access the long mode and pukes.

The moral of the story here is that if you attempt to nest Server 2003 -> ESX -> Workstation, stick with the x86 versions for now.