Thank god the beta has gone public, because for the past 6 months that I have been on the private release program, I have been dying to get this into the community at large.
Workstation has undergone significant changes in 6.5, one of which is that it is no longer possible to run ESX as a VM. See this thread in the VMware forums for details, and I urge anyone else who cares to request the feature be brought back like I did in the 7th post of that thread :-)