VMware has removed the option to run ESX 3.5 on Workstation 6.5 but there is some light at the horizon. Jim Mattson over at VMware didn’t make a promise but the final version of Workstation 6.5 might include support for ESX 3.5, in the beta I’m using at the moment it didn’t work tough.
The code to support this mode of execution no longer exists. However, as kma said earlier, "please stay tuned." No promises, but Workstation 6.5 is not fully cooked yet, and something may yet appear that suits your needs.