I used Exchange 2003 and it has a Recovery Storage Group, which allows an Admin to restore a DB to the RSG and from there use Exmerge to restore individual mailboxes or items.
However I have never had a chance to use, since we had CommVault backup solution, and the restore of individual mailboxes or even messages can be done staright from CommVault backup to the Exchange 2003 DB.
I have heard of Exchange 2010 RDB, it sounds like it is the same. But nowadays most companies have good backup solutions, I wonder if we need this RDB or it comes into play only if you don't have a backup solution such as CommVault, BackupExce, NetBackup...??