When trying to migrate from vCenter 2.5 to vCenter 5, I removed an ESX 3.5 host from vC2.5, and attempted to manage it with my new vC5 server. I received this error message, and the attempt fails:
VI3 hosts cannot be added if vCenter is licensed but License Server is not Configured
(VI3 refers to Virtual Infrastructure 3: ESX 3.x and vCenter 2.5. Most significantly in this context: their special, special license server that installs with vCenter 2.5)
My searches found examples of people having to install the old VI3 license server on their vC5 servers, but I still have a working VI3 license server with all of the licenses I need on the vC2.5 box.