Will promox even actually let you create a cluster across to different data centres?
Obviously unless you somehow link them over L2TP or some other form of VPN so the IP's can route to the other box it's likely that Migrate wouldn't work but it might still be useful for the ablity to "see" the VM's on the other boxes from a single location (I'm assuming of course that once clustered you can start/stop/view any VM on the cluster)