The cause is due to UUID of ESX host got changed after upgrade from vSphere. Please refer to Primus emc309039 at support.emc.com for resolution. 
Upgraded my Environment to vCenter 5.1 and ESXi 5.1 Since the upgrade, I can see my vCenter being registerred on my VNXe, but not the hosts that are managed by this vCenter and connected to the VNXe  Is this a known issue, or a bug, or does VNXe simply not support vSphere 5.1 yet?  Duco
vCenter 5.1 server registered on VNXe does not show ESXi 5.1 hosts