AO, Interestingly, after reading your post, I had a look at my photos on my iPad 2 and noticed that they were no longer being synched. I had run out of space a few months back so maybe that is when it stopped synching similar to your case. Despite not synching photos, there were still 10gb of photos on the device so I tried to remove them.
When I deselected synching pictures and completed a sync, my iPad still showed a couple of gigs of photos still on the device. I had to restart the ipad and then do another synch removing photos before I could get rid of them all. Once I had got rid of them all I could then do a selective sync.
It may not be the same as your issue and if so then I am out of ideas!