In the beta, DK damage was nerfed a few times.
They didn't really have a lot of tanking data, as everyone had fairly low-end gear on the premades or random gear from drops.
Not saying they had no data, just not the best they could have had.

In my opinion, they let DK's enter the live world a bit too strong, because they wanted the class to be initially powerful and kick ass.
The power will gradually (over the course of arena) be scaled back a little, because they have to be at least in the same bracket as other classes.
Otherwise arena's would have 40-50% DK representation, and even if things aren't totally balanced they want a more even spread of classes.
The game is balanced around 5's arena, so depending how they do as the season progresses will determine if they get buffs (probably to pve tanking) or nerfs (pvp damage/mitigation).