Unity wont care if you're moving the message store to a storage farm... we're gaining access to the mailstore via MAPI and the Exchange interface will insulate us from that...
Even if you move users around to other storage groups on other Exchange servers in the routing group, Unity will "follow" the user's mailboxes around... when we attempt to log into a mailbox and it fails, we back out and ask AD where this guy's mailbox is now and then log in again. So the first time a user logs in over the phone there may be a bit of a delay while this happens but other than that you wouldn't notice an issue.
