So according to a tweet from Holinka on the first page there's a new mount (one for horde, one for alliance) coming in 5.4 for PvPers and the criteria for this achievement is:
100 3v3 wins
40 RBG wins
which could of course change before the patch, but apparantly that's what it is right now.
I personally think this is a major leap forward in getting more people into arenas and getting the queues more active again. People of all skill levels will be entering the arena to go for this mount, I'd love to see the number of 3v3 wins raised (or a seperate achievement) to say, 500 or 1000 because 100 is really easy to get in a 6+month season, even for the most casual of players. But still it's a great addition!