^ That was an April Fool's Joke that blew up over last weekend (Cameron isn't directing a sequel).
Cameron made a off hand joke about it on the red carpet for Titantic which was taken out of context. Still he and Scott apparently talked about Prometheus, specifically toward VFX and 3D film. It was in reference to those discussions that Cameron made the joke about Fox needing to wait a while, i.e. after he was finished with the Avatar sequels, if they wanted him to direct it.