http://blog.coildomain.com/2011/08/vcenter-5-the-fqdn-cannot-be-resolved/

I had the same problem, but my reverse lookup record was fine. It took 30 minutes of hair-pulling, but I figured out that the binding order of the NICs on my vCenter server was causing it. I have one NIC on the server network and another for iSCSI connectivity. Since the iSCSI adapter was bound first, the FQDN check kept failing since the iSCSI network is isolated from the server networks, and of course, the DNS server.

To fix this if you have a similar situation, goto the Change Adapter Settings screen where it lists your local area connections. Hold the ALT key for a second and then goto advanced and advanced settings. Under Connections, move the connection that has access to the DNS server to the top.

http://theregime.wordpress.com/2008/03/04/how-to-setview-the-nic-bind-order-in-windows/