iTunes could not back up the iPhone because the backup could not be saved on the computer
Had the same issue with my 4S, and it was fine before.  Found the solution here:
http://support.apple.com/kb/TS2529?viewlocale=en_US&amp;locale=en_US

Basically had to copy my last backup to my homepage (drag/drop), delete the old backups (drag to trash), and then Sync and backup worked fine in iTunes.