Cameron pissed off all of Hollywood when he actually USED that as the mineral causing the strife in Pocahontas Avatar.
"Unobtanium" is a movie development standard placeholder (like "MacGuffin) in scripts or treatments, usually replaced by some name at the last minute.
By actually USING the term, Cameron took that away and I think Hollywood is still looking for an alternative.
Engineers have been using the term for years, as in, "That thing won't fly unless you make it of pure unobtanium."