As of Exchange 2003 SP1, Exchange has the ability to restore backups to a Recovery Storage Group (RSG) in order to retrieve a lost mailbox, or as part of retrieving lost mail from a mailbox. In many situations it is a preferred method to doing brick-level backups for the purpose of finer-grained restores. There does, however, appear to be confusion as to how this works, specifically with the Exchange plugin.
One note of caution at this point: make sure the option that this database can be overwritten by a restore is NOT CHECKED in all Storage Groups other than your Recovery Storage Group. This cannot be stressed enough, as failure to do so may cause loss of data. This setting is usually unchecked by default, but double-check this anyway.
To create the Recovery Storage Group, in Exchange System Manager, right-click on the server, select New and then Recovery Storage Group. Create the logs and system paths in locations that have sufficient space to hold the .edb and .stm files for your restore. Right-click on the Recovery Storage Group and select Add Database to Recover. Select the Mailbox stores you're after. Make sure that the store is not mounted (a little red down arrow icon in the System Manager tree indicates this), and that it is set to allow overwrites by restores - the option referred to above. Contrary to your other Storage Group mailbox stores, in your Recovery Storage Group mailbox stores, you want this option to be CHECKED (which is also the default setting).
Nothing else needs to be configured. Exchange knows that restores done to the Exchange server are to be restored to the Recovery Storage Group simply by virtue of the fact that a Recovery Storage Group exists.
Use the Exchange plugin to indicate which backup you wish to restore, select your mailbox stores, and be sure to deselect any public folder stores, since this will cause the restore attempt to fail. Recovery Storage Groups can only contain mailbox stores.