Of course many of Cameron's ideas came from somewhere. He grew up steeped in sci-fi, and at some point probably read Poul Anderson's story "Call Me Joe," first published in 1957. He says he doesn't remember it, and that they scanned his script--that's industry-speak for checking to see if some other published work shares aspects of his story--and didn't find anything. "You can't go back more than 50 years," he says, with some exasperation.
Like The Terminator, Avatar grew out of the rich smorgasbord of material Cameron had ingested: sci-fi, New World mythology and Edgar Rice Burroughs' Martian novels.
It's pretty easy for someone to write up a synopsis of Disney's Pocahontas and annotate it. (The official Disney synopsis, which I have, is quite different.) This synopsis from Matt Bateman is pretty accurate. And the joke works well up to a point. But there's much more going on than this in Avatar.
Cameron must be doing something right. Avatar introduces moviegoers to an immersive new world, a Utopia (or Oz, as Steven Spielberg describes it--hence the conscious reference to "we're not in Kansas anymore") where they indulge the fantasy of being one with nature, able to swoop and fly. I can't wait to go back there.