I have a vCenter server 4.0 and I went to upgrade one of my host machines. They were all running 3.5. The upgrade could not have been easier using the vCenter upgrade manager. Once the host rebooted it would not stay connected to the vCenter server. I could not figure it out the remaining 3.5 hosts stayed connected just fine.
Did some searching turned out I needed to add the IP address of the vCenter server into the Managed IP Address field under vCenter Server Settings.
See this article. http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011647
