Until they come out with more info, based on what little I have read, I think that unless there is a mob type we are also good against. We will fill more of a secendary tank role one raids for debuffing and CC.
When it comes to debuffing only one DK may be useful on a raid but our CC ability may make more then one DK wanted.
More or less same thing goes for group to a lesser degree. Unless they make us either pretty resistant to somthing the other tanks are not resistant to or make us better against some brand of mob, I see a secondary role for us.
Of course this is all guess work from limited info about the DK.
Who knows, they could be holding back info just cause we are soooo damn cool!
Haruchai