This was a drop from Mith'rethis the Enchanter, whom prior to Cataclysm was a level 55 rare elite. Since a mob can never drop an item that is higher in level than its source -- and no one on Wowhead has claimed to have obtained it after Patch 4.0.3a -- this robe is unobtainable. If you see it on the AH, expect it to command a very high price.
Particularly because of its model and skin, too, not just its scarcity.
It seems that these robes are once again obtainable, albeit at a very much lower rate compared to the original source.