Unobtanium has been used by the science community for a long time (the word that is) http://en.wikipedia.org/wiki/Unobtainium
James Cameron has mentioned that SCI-FI movies often suspend the laws of physics in order to tell their story. He mentioned that the floating mountains would require a magnetic field so strong that it would "rip the hemoglobin out of your blood". http://aintitcool.com/node/43923
I haven't seen the movie so I can't comment. But I think that's an inaccurate and grossly negative view. I thought it was odd that he eventually became their leader, since he could never really be a Na'vi to the core, he didn't grow up as one. But other than that I think it makes a good point.
