For a look at more global arena statistics, you guys should check out:
http://www.arenajunkies.com/rankings/

Check out the amount of Chinese teams at the top lol

To OP:
If you're interested in arenas, druid would be a good choice because just about every popular team setup in 3v3, besides RMP (rogue, mage, priest), has a druid in it.
As a mage, if you follow American trends, your team choices are more limited, basically limited to RMP if you want to be at the top.
If you look at Chinese teams though, you can see they have a lot of WMDs.

Anyway, the expansion will probably change everything, so just play what you like more lol.

haha yeah, I do go to Ming's blog. Its a pretty good place to check up on latest arena/tournament news.
on WMDs, I'd like to see China use WMDs against US in tournys lol


It's hard to say which classes are good to roll as now since expansion is in like 2 months.
ATM the strongest/most popular arena classes are druids, rogues, warriors.. locks, mages and priests do fine,
though mages and priests are usually stuck with RMP. Paladins, hunters and shamans are almost non existent in 3v3 though paladins still have a place in 5v5.
3v3 might be faster for arena points then 5v5 though, shorter queues and a lot easier to get 2200-2300 rated.

For 3v3s, the most popular comps in the US are WLD (Warr, warLock, Druid), RMP (Rogue, Mage, Priest), RLD (Rogue, Lock, Druid) and cleave team which is 2 melee - Rogue, Warr, Druid..
When you're at the top, most of the time you'll prolly face these comps. But as long as you play a reasonable comp that's not retarded, 2000 rating should still be no problem.