It's a limitation of a given host, ESX or otherwise. The 256 LUN limit applies to each ESX host, but since an RDM needs to be presented to all ESX hosts, the limit is effectively per cluster. It's also a concern here, but so far I've drawn a blank as far as finding a solution (other than creating a new cluster, that is!!). If anyone has any solutions, I'd love to hear them - I know we're not alone!!