Shadowpriests are looking like a viable arena component with a "Oh shit" ability (dispersion), 60% less SW
backlash and AoE horror scream (making fear breaks far less effective). They will be a powerful CC arena class and will stack up well with other CC heavy classes (Rogues, Mages, Druids). Rogue/SPriest looks to still be very potent in 2v2 with both classes having immunity abilities and various forms of CC. Mage/Rogue/Spriest looks to be a nasty CC combo triple dps team, especially considering all three classes posses s"Oh shit" buttons.
Considering the fact that a variation of Holy and Disc priests are always one of (though usually not the best) healer in all PvP brackets, Priest looks like a good choice for Wotlk arena, as either Shadow or Healer.
Keep in mind that while most classes are viable in all PvP brackets some are far more reliant on a few setups. For example Hunters are amazing in 3v3, if paired with a Disc Priest and Resto Druid, while many other setups are not as effective. Wheras Rogue, Warriors and Druids, many setups in 3v3 are effective, and thus your pool of potential arena partners is much greater.
If you want to pick a potent arena class that can fit into multiple setups (in 2v2 and 3v3 in particular) I would say go with a Druid, Rogue or Warrior. These classes just have strong base mechanics and will always be able to find a good arena setup. Avoid Mages and Hunters in particular, as they lack the flexability for many types of arena setups.