Like the vSphere 4.0 version the install process works fine and starts the vCenter service. After the sucessfull vCenter installation i added my both ESXi Server to the new cluster. But after a few seconds the first ESXi lost the connection to the vCenter Server! After a few more the second ESXi lost the connection. After several reconnections this happens all the time.
So i began to look for the firewall rules and there where several rules for the vCenter services, but... only for the domain membership!
So, with this knowledge i bring the vCenter Server into the domain, activating the firewall, check the rules and everything was fine.
UPDATE: Today I had the same problem with ESXi 5 hosts and a vCenter 5 Server. All software firewalls were deactivated... but the network guys didn´t configure the ports as expected. So if your vCenter Server loses connection take a look into the Switch- and Firewall-Configs :)