actually unobtainium is an inside joke from the movie industry and shows just how lazy the writing was in avatar: Don't get me wrong it was very pretty, well acted, well designed & had a lot of emotional resonance, but it was really really really badly written. By now 2 out of every 3 readers of this statement have jumped on the respond button to tell me how wrong i am... but before you do, go back and watch the movie again. This time take in the actual writing rather then the pretty pictures.
Back to my main point. Unobtainium is an inside joke from the movie industry & is a term used to describe any plot breaking substance that a movies base concept is built around. Consider it to be the furry little critter in gremlins (because those rules did not make sense), the microchips in small soldiers or the crystal growing crystal in Superman Returns. all these are examples of unobtainium.
-M