Druids still top the charts for 2v2
The site Vhairi's Scratchpad has posted some very interesting analysis of the arenas in the World of Warcraft. They show how each of the three sizes of arena team battles break out based on character selection. They also chart the class makeup of teams at each arena rating. It clearly shows that several classes are far over or under represented in each category as you move up the ratings.
You can find all the details here.