I have the same problem with my iPad 1. it backed up ok until March 2011 but since then i get the same error message
It can sync and copy purchases ok but cannot backup and thus I cannot upgrade beyond os 4.2.1.
I have deleted backups in iTunes preferences, deleted in the library / application support / mobilesync / backup folder ( then restored this from an old timemachine backup), deleted the iPod Photo Cache many times....
If I just use the backup command from the side bar, I can backup until the progress indicator reaches 80% in the progress indicator but then the error message appears. When I sync the backup only gets 10% along then moves onto the next sync activity.
I've ordered a new iPad2 to be delivered mid May - seems that I will have to wait until it arrives, setup a new iPad then redo all the settings and apps by just looking at the old iPad and recreating them.
My iPhone 4 syncs and backs up ok and is running 4.3 ok. Just the iPad has a problem.