Well, they did sort of address it in Avatar.
The antigravity element was a naturally-occurring in the world. Masses that contained high concentrations would float, masses that did not would stay on the ground. Obviously, Unobtanium was not ingested or retained by their bodies in large quantities.
My sky fantasies (illustrations in my case) involve vast blimplike floating constructs or creatures, upon which people, flora and fauna cling and live out their lives.