Since I bought my iPad 2, I have consistently received an error message that the sync didn't work because the 'required file can not be found.' It seemed like everything was syncing OK with my iMac so I didn't worry about it.
However, when I tried to add new photos to my iPad 2, the syncing issue became a problem. Turns out this problem has to do with syncing photos and the iPod photo cache. After some Googling, I found the solution, which I'm passing along here:
1. Shut down iTunes.
2. Open the Pictures folder in the Mac finder.
3. Right click on the iPhoto library and click on 'show package contents.'
4. Find the 'iPod photo cache' file and drag to desktop or delete it. (A new file will be created automatically later.)
5. Close the finder.
6. Reopen iTunes and sync your iPad.
7. The iPod photo cache rebuilds itself and your problem should be solved.
You can read comments about this solution in the Apple Support forum.