I am having trouble backing up my iphone 3GS to my computer (and also to icloud). After the sync completes, I get a message stating that the back up was not done because the back up failed. I've never had a problem before with this.
Try going into iTunes and deleting past backups. Many times that will fix this error.
You can also check this support document http://support.apple.com/kb/TS2529 regarding backups.
Retrieving data ...