Earlier in Cataclysm, the changes to the tanking mechanic, Vengeance, led to tanking specs suddenly becoming viable damage dealers in PvP. This was removed in 4.3, apparently because it was felt that tanks were too powerful, given their survivability in PvP. Fast forward a few months, and we find some vestiges of PvP in tank specs remain. In mid-level Arena, blood DKs continue to crop up every now and then on survival-centric teams; however, they are not really the most formidable opponent. Their damage output is not so high, and if kited, they can be fairly easily (if slowly) taken down.
Many a feral cat druid will shift into bear form to use Frenzied Regeneration and increase their survival, and bear damage is fairly high (for a tank class, at least). However, bears lack the high self-healing of a blood DK, since Vengeance's removal.
Should Vengeance be returned to PvP? Frankly, I don't think so. Does tanking have a place in PvP? Should anyone PvP as a tank? Does tanking even exist in PvP?
Well, let's have a think about that. What is tanking? Tanking, in a PvE sense, is the act of ensuring the enemy is attacking you and not your weaker allies. If a PvE enemy is attacking a healer, the tank is doing something wrong.