While working on an upgrade to vSphere 5.0U1 on a Cisco UCS environment, where the ESX hosts boot from SAN, I noticed one of the hosts was not registered correctly on the EMC VNX, as it showed up as unmanaged. Because the ESX hosts boot from SAN, the host has to be registered before it can auto register, and when it was registered manually the host was not able to update the registration.
I tried several things, like power off hosts, deregister initiators, power on host, reregister boot initiators and add registered host to storage group again, and reboot host, with and without restarting the VNX management servers, before I found a solution that did work, which is kind of dirty if you ask me.
The solution is to put the ESX host in maintenance mode, enter engineering mode on the VNX, and deregister all logged in initiators for that host. Next, do a rescan on the host (which will not work because the deregistration removed the host from the Storage Group) to let the host register at the VNX, and add the newly registerred host to the correct Storage Group on the VNX again. Do an other rescan on the ESX host now to see your Datastores again.
IMPORTANT: Reboot your host after you did this, since a rescan of the Datastores does not Rescan your boot disk if you use Boot from SAN
Please remember this is could be a tricky operation since even though the host is in maintenance mode, you still yank the mounted LUNs from the host while it is running, which could cause an APD condition for instance, and the ESX host can probably only survive without its boot LUN for a limited amount of time. Unmounting the Datastores on the host in ESX 5 before deregistering could be a cleaner alternative, but have not tested that.