My DS, 2.8, has constantly had his hands in his nappy for weeks. I don't believe he is actually scratching, and his bottom looks completely normal to me (not inflamed or red or anything). So far, I've put this down to being "just a phase...".
Dr. Google (I know...) suggests that it could be threadworms. If your child has had those, what were the symptoms? Did their bottom look any different?