A while back, a few of us figured out how to get the USB key with 3i beta (given away at VMworld) to run inside VMware Workstation:
http://communities.vmware.com/message/754336#754336

Now that ESX 3.5 is out, I see threads starting up about installing on a SCSI drive inside VMware Workstation:
http://communities.vmware.com/thread/116639?tstart=0

I cloned the 3i VM in Workstation, then with that clone, I added a SCSI disk and mounted the iso of the GA level of ESXS 3.5. After accepting is offer to change the disk type, I was able to then install and run ESX 3.5 from within the VM just fine.

I've attached a copy of my working vmx file.

Only tested for a few minutes, and a few gotchas already:
when I downloaded a virtual appliance right from its welcome page, it crashed ESX when the virtual appliance was powered on
when I pointed another Workstation VM (XP), one that had VIC 2.0.2 U2, I uninstalled, then pointed browser to bridged-networking IP of ESX 3.5 and installed new VIC 3.5, but it came up with an error at the end. However, it actually seemed to work fine, launched from desktop icon it created.

I hope this helps steer others in the right direction, and the speed/responsiveness was far better than I ever had the 3i working fyi.

Next steps, make another W2K3SP2 VM and install SQL 2005 and VirtualCenter 2.5 with VUM...also inside VMware Workstation (for portability and training and demonstration purposes at bandwidth constrained customer sites, and for "fun")