Sorry in advance for my bad english.

I'm playing my DK now since the release of wotlk. As many people before have said, tanking without frost-presence is a suicid. If you go ahead and tank that mob, make sure that you have at least a defense of 530. The defense needs to be higher as the mob-level increases. You'll need 540 defense against bosses in naxxramas. I rather have 1-2 more points in defense to avoid any problems due to scaling against any oponent. It has happen before that a tank got one-hitted from a boss with a critical hit, while he had 540 defense. WoW didnt calculate everything correctly and so he had like 1% chance on getting a critical hit. Just a few basic-stats from my DK: 27727hp, 18.44% parry, 27.49% dodge,16899 armor and 542 defense. Stats are messured in blood-presence. Talents are set to 7-57-7.

Tanking in frost-presence will give you 2 bonuses. An increased armor and more hp. You can put your talents in every skill-tree as you like as long as you chose talents with defensive bonuses.