I enjoy the Halo series, and although admitedly I haven't seen Avatar, I see no connection between the trailers, or what I have read about Avatar, and Halo.
More to the point, this thread has been going for three pages now, and not one person has defended the idea that Avatar is ripping off Halo. Since the Escapist is a gaming website, its stands to reason that if a significant amount of people were claiming the ripoff angle, it would have shown up several times in this thread by now.
It seems to me that Cameron is just making up a phony argument, to get publicity. If someone made a mobster flick, and then went around saying that people were complaining that it was a ripoff of the Godfather, when no one had made that connection, the intention would be obvious. The idea is to draw even more attention to your product, by making up phony controversies.