If you add an ESX 3.5 host to your vSphere Cluster,there is a change you’ll get this warning:
The solution is simple, you need the “old” license server running in your network. So download the “old” license server from http://vmware.com/download/vi/drivers_tools.html
Install the License Server for ESX 3.5 on different server than your vCenter box. I tried installing it on the same box, but it ended with a failed startup of the License server services. The installation is straight forward (read next, next, add license file, next and finish).
After the installation open the vCenter Service Settings option.
And enter the address of your license server and press ok.
Now it’s possible to add the ESX 3.5 host to your cluster.