If mages had ANY real boxing survivability in arenas they might be worth playing. They can't heal - shammys can. A single point of weakness (only one healer) in arenas is.... problematic, to say the least.

As it stands now, mages are not likely to do well in WOTLK arenas. That said, in large scale PvP (hi 2 u Wintergrasp/AV) they will most likely, as always, dominate.