Okay, now I'm just starting to throw out ideas...
- Try rebooting your computer (and thus, iTunes)
- Restart or reset your iPad (or both!), then try a back-up again
- Go into your computer's directory and delete any backups that might be there (sometimes, iTunes misses one...). Here are some general addresses where iTunes puts the backups (yours may be a bit different):
* Mac: ~/Library/Application Support/MobileSync/Backup/
* Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
* Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
Those are the steps that I would do, if faced with your problem. If none of these work, then I'm sorry, but I'm out of ideas and all I can suggest is you visit an Apple store and talk to the geniuses there. Sorry.
Marilyn