I just updated to iOS 4.3 on my iPhone, which is connected to a MacBook Pro, and when the installation was done, I tried to sync. The sync stalled at step four of four on syncing photos, with a message telling me that the sync could not complete because "The required file cannot be found."
I called Apple right away, and the support rep, after checking with a few higher levels of tech support, asked me to uncheck the box to sync photos. The sync worked. I was then told that the problem had to be with one of the photos in my iPhoto library, and I would have to find it using the process of elimination. This didn't sound like fun since I have over 3600 photos, and this would take quite a long time to accomplish.
I tried to find a simpler solution and found it. Incorrectly thinking that it might be in the last import, I had my iPhone 3GS sync all events except the last one. The sync worked flawlessly. Then I added the last event, and the sync still worked. Next I added all the albums and faces, and the sync worked again. I girded my loins and decided to try to sync all photos, events and faces ... and was able to sync perfectly.
My iPad had the same issue, but first unchecking syncing and then re-syncing everything (which took a while, as all of the photos were re-optimized and copied over) worked just fine. So, at least for me, there was a problem with syncing photos, and in both cases, deleting the photos and then having the computers and iDevices sync all photos, events and faces solved the problem.
I don't know how widespread this problem might be, or if it even affected anyone but me, but if you are in the same boat, give this a try.