DK DPS really shines in AE, they usually annihilate the meters on trash packs. On bosses though, they're fairly lack-lustre. They're alright, and they bring other buffs, but they don't touch fury warriors or other high DPS classes.
DK tanks are fine, I imagine. Ours went inactive just before i started raiding, so i haven't seen them in action, but they're perfectly viable, a lot of guilds use them.
I'm not sure what you mean by warriors being screwed as they can be replaced by paladins. You can replace a DK tank with a paladin too. You could replace any tank with any other, IMO.
I haven't seen a fight which requires a specific tank yet, Blizzard have "dumbed down" raiding so it doesn't require specific classes for anything really.
And yes, Frost spec seems to be popular for tanking, Blood seems popular for leveling (lots of self-healing, minimal downtime), and Unholy seems best for AE dps.