Have you guys considered using Perdition to proxy/redirect IMAP/POP to distribute backend storage to as many as backends as you want? You wont need any SAN/NAS/NFS/GFS/whatever, just a user/storage mapping (ldap, mysql, etc...), and you might even add spare or active/active servers to each storage with DRDB or something alike.
Just my 2 cents. Deives
