Here's something like what we're thinking of. This is from SugarSync, and isn't totally applicable, but it will serve the purpose. Files that are synced have the green checkmark. Files that are in progress get the yellow icon with the two arrows chasing each other:
Mr. Yuck? Well, OK, he's not part of SugarSync. I just added him in to see if you were paying attention. He, or something like him, could be used as an indicator to show there's a problem (like a backup failure) for a particular file.