GFS instead of clustering?
Would it be possible to make Zimbra utilise the Global File System (GFS) from RedHat in order to allow multiple backend servers simultanious access to the message store?
If used with a SAN, iSCSI and some form of layer-4 load balancing, you could eliminate the need to run clustering services to get redundancy. You then don't have to try and split mailboxes across muliple stores in order to try and get quasi load balancing. You can scale the system just by adding an additional backend server without having to try and shuffle mailboxes around stores.