In short, my opinion of DK tanking is that you have 3 talents for avoidance/mitigation. The others are all for threat. You have a few in each of the three trees that help with survivability (bone shield, shadow of death are the two in the unholy tree). Here's my spec:
The World of Warcraft Armory
I'm considering moving the three points from Morbidity into BCB for even more single target dps, but I haven't decided to do it quite yet. While I am main tank, so my primary responsibility is single target threat, I do have the responsibility of being able to hold some threat on the trash and without an almost continuos D&D, I'm not sure I'd be able to do it on some of the longer lasting aoe trash pulls.