It is extremely unlikely that it is, because Parker Selfridge holds a piece of it up in the film without any protection. It may be slightly, but it doesn't appear to affect humans or the Na'vi. "Unobtanium" is a name given to materials that are rare or difficult to find. [For example in the movie "The Core" [Aaron Ekheart, Hilary Swank] Unobtanium is what the ships, Virgil's, shell shaped hull material is called but its true name is 37 syllables long.] Avatar too, the Super-conductor material was intended to recieve a more appropriate name, but "Unobtanium" had been used for so long that it stuck. This was written somewhere on A site for Avatar I'm sure but I can't recall where now. So despite the similarity to names of Radioactive isotopes like Uranium, Plutonium and Radon it is possible that it is not.