Caster has posted his "first successful pull" of an exhaustive list of arena PvP stats about class and spec over on the forums. There are tons of stats to play around with here-- for each bracket of 5v5, 3v3, and 2v2, he's got the average, minimum, and maximum of specs for each class, wins and losses, the low, average, and high of hit points, and even a breakdown of who's grouping with who.

Not surprisingly (though I was surprised back when Arenas first started up), Warriors and Paladins are topping the charts in terms of numbers, so it seems a good way to win 5v5 Arenas is simply to stay alive. Apparently 2v2 teams are the most volatile (since people switch around with them all the time), but the numbers there are the most interesting-- Warlocks and Priests top the charts there (thanks a lot, Fear), and it turns out Druids might have actually have a place in Arena PvP-- as long as it's right alongside a Lock.

And the big loser is apparently Hunters-- their numbers are down to single digits in 2v2 and 3v3 (as in 1 and 2 total), which means players are not taking their Hunters into the arenas. Is that because Hunters aren't useful or viable enough in the smaller teams? I'll leave that one up to you.