I believe Unobtanium is the proper way of spelling, and James Cameron agrees with this spelling.
Wikipedia spells it as unobtainium
http://en.wikipedia.org/wiki/UnobtainiumThough I am 100% fine with the spelling of Unobtanium, it differentiates it from Unobtainium which other people use for other things. Unobtanium is a coin. Unobtainium is whatever else.