I just thought I'd give my spec in so to see if it works for any of you, I hate death knights (although i am one) because it feels like all the scrub hunters re-rolled them lol, there are alot of bads out there so any help i can give to get our class out of this hole is worth it. My current spec is Frost i tank Ulduar 25 man easy with it (never mind my blood spec I was just testing and it was meh threat) atm in Ulduar i pull 4k threat when i have bad luck and get no chance on hits and rune strikes but when i get on a roll its a constant 6-7k but also spikes to 8-9k. My spec works well for AoE threat and solo target threat, I haven't found a better threat spec yet (I can say this hold good AoE threat since I'm able to tank Thorim in the arena quite easily when i have locks and mages who pull about 6k AoE threat).
I hope my spec helps anyone having problems with death knight tanking, and if your failing i suggest re-rolling paladin or hunter : P
Also if I'm online and you really want to ask me questions in game you can feel free to pst me and unless I'm afk I'll get back to you.
The World of Warcraft Armory
(trinket is bad because main tank of guild ninja's all the good ones >.< lol)