An attempt to prepare the storage group '' for backup failed because the storage group is already in the process of being backed up. The error code is

This event is logged when a VSS backup operation fails in Exchange 2007, because: 
1. VSS uses the Microsoft Jet database engine to freeze or thaw a snapshot backup.
2. The Jet database engine encounters an error.
3. The Jet database engine silently frees all storage groups from the snapshot session when the error occurs.

Solution?  Dismount the Stores and restart the Information Store service....

Really?  Come on Microsoft, I'm going to dismount my stores and restart the information store service before I backup my database every night?  I don't think so...