New VMware Workstation 6 support running ESX 3.5 now!
It appears VMware has listened to the customers and they are putting back in the ability to run VMware ESX 3.5 inside of VMware Workstation. Reports so far are detailed in the links below but it appears at present you can installed ESX 3.5 inside workstation but you can not run any virtual machines yet...with that support coming in the next beta release.
There is also some changes needed in your VMX file. See these links: