If you've gotten all excited for iOS 4.3's great new features only to find that it won't let you sync with iTunes, weblog TUAW might have the solution: just re-sync your photo library.
We're not sure how widespread this problem is (I personally haven't had a chance to upgrade to iOS 4.3 yet), but it looks like a few people are experiencing a "required file cannot be found" issue when trying to sync with iTunes after updating to iOS 4.3. The culprit, it seems, is your photo library. You can either delete the iPod or iPhone photo cache from
~/Pictures/iPhoto Library/ (you'll have to right click on it and hit "Show Package Contents"), or uncheck photo syncing in iTunes and let it re-sync all your photos. This will take a while, since it needs to re-optimise and re-sync everything, but you should find that it actually finishes the sync after it's done so. Hit the link for more info, and let us know if this solution works for you in the comments.