Actually, I just found an email from Todd Stone from about a month ago on an internal alias describing the new policy that will be outlined in the next version of the Unity Design Guide:
"The stated limit for off-box mailstores has been revised to remove the maximum limit of 5 mailstores. This limit was initially stated in the Unity Design Guide. The intent was to align with the highest number of offbox mailstores known to be tested with Unity. The upcoming version of the Cisco Unity Design Guide will now reflect a recommended maximum of 10 mailstores. Note, this is a recommendation and not a requirement, but a solutions designer is strongly urged to comply with this recommendation. The recommendation is based on limiting the administrative overhead of managing subscribers on 10 mailstores. It is important to note that the off-box mailstore limits may be revised in the future as necessary. It is also important to note that any Unity server is limited to supporting the total number of subscribers that it can service based on the hardware platform it is installed onto.
This policy does not mean that Unity can service an unlimited number of mailstores. In fact, attempting to have a single Unity server or failover pair service too many off-box mailstores is impractical and isn't sustainable. Care should be taken in the way any Unity server is designed to service off-box mailstores within a given messaging infrastructure. In addition, while Unity hardware platforms may state a maximum number of subscribers that can be housed on the platform, it isn't practical to place the maximum number of subscribers on the server unless the proper capacity planning has been performed and consideration has been made to install additional Unity servers to support growth.
For each version of a mailstore that Unity supports, the best description of a mailstore is the physical server in which the mailstore resides. This may actually be an Exchange 2000 or Domino cluster. This shouldn't be confused with the descriptions for mailbox storage for each different version of messaging system that Unity supports."
And in the summary:
"The real or primary limitation for Unity is the total number of subscribers that can be supported by the hardware platform that Unity is being installed onto. It is recommended that you prevent Unity from being maxed out; in other words, just because Unity supports a maximum of 7500 subscribers on a given server, don't put 7500 subscribers on the server. Save some space in case you need to make changes or in case of unexpected growth.
....
There are other factors that make up a given Unity configuration. The total number of offbox stores is just one aspect in designing a solution. Another important aspect of offbox mailstore support is that Unity must be co-located with any and all mailstores it is servicing. Unity cannot be remotely connected to a mailstore, especially via a WAN connection. For more information see the upcoming revision of the Cisco Unity Design Guide or contact a Unity TME."
