For raid bosses Warriors are still far and away the preferred choice of tank. I've talked to Paladins and Druids that have tanked everything up to and including Illidan and even they say they are not as good as Warriors.border said:Warriors have shared their tanking role with Druids and Paladins for nearly 18 months now. All the tank-capable classes have specific utility that is unique, though. Death Knight will be much the same.
Death Knight tanks will start off as the "heavy magic boss tank", time will tell if they move out of that niche though.
Melee DPS is another story all together however.