Judging from what Blizzard said at the WWI, each tree will be able to tank. While they also said not to theorycraft from the alpha builds, I'll still do it. Frost looks to be the best for tanking at the moment. Blood is good for pvp/pve, Unholy i think is going to be the most versatile spec. Unholy Blight basically gives you the opportunity of being able to multi-tank by spamming Blood Boil without spending lots of time trying to apply diseases to all those targets.
And for DPS... drop that dumb *** notion of notion of Dual Wielding. Dual Wielding is going to be for tanking, period. Just look into the frost tree... it has the option for offhand damage increased. Also, the abilities are about putting up debuffs on the target. Frozen Rune weapon on both weapons will mean more threat... Also keep in mind, how many 2handed swords did you see that had +parry, +defense and armor and all that on them? Exactly... But since DKs can now use maces, axes, and swords... that kind of grants them the ability to dual wield all the tank weapons in the game. So it helps them gear out for tanking instead of farming that one instance for that one special 2hand tank weapon drop. Itemization will be easier for them since they're going to have a hard time focusing on parry and def gear. Lot of the tank gear has shield block and block rating on it that does nothing for a DK. DW will be for tanking not DPS.
Another thing, pretty much ALL of the DK's strikes are 60% with some added effect. Heart Strike, Frost Strike, and Plague Strike are the only ones that do more than 60% weapon damage. And Plague strike is the only one that is not a talent. So you'd be wasting a rune on a strike that would deal only 60% weapon damage from a 1h weapon. There's a reason why MS wars use 2handers... You might be able to make some hodge podge spec with DW, but you need to dump at the very least 13 points into Frost. But if you go that route, you should put a lot more since Frost abilities synergize so well with mages. I dont see DW being the top DPS choice but I think it will certainly be a good choice. The problem is that you have to gear for +hit way more than a 2hander.
Unholy and Blood will be close in terms of PvE damage. Plague strike looks like it's going to be the Crusader Strike/MS equivalent for DKs