This is interesting - I've also heard several referencies to third sets of teeth over the years. I also know 1 person who has grown some of a third set of teeth.
One article I read stated that Australian Aboriginies often grow a third set of teeth later in life and that other racial groups have been known to too. I wish I'd made a note of the article, but I'll keep my eyes open for future reference!
The video I have about Body Electronics mentions changes to the jaw and skull. I wondered what effect it would have on teeth.