So how do you spec for tanking as a Death Knight? Other tanking classes just have one tree for tanking, Death Knights have 3. Each has their own niche, and each can perform equally well given ideal circumstances. Obviously some will be better in one area than another, but the idea is you pick whichever one suits your playstyle.
Personally I have used all 3 of the trees. I started out as Blood for 5 mans, and then switched to Unholy pre-Bone Shield nerf. After the nerf I stayed as Unholy for a little bit, and then went back to Blood for a long time. Now that we are working on Sarth + 2, I've gone back to Frost for Whelp tanking.
Any Death Knight tanking spec will start out with the 3 base tanking talents at the bottom of each tree - Blade Barrier, Toughness, and Anticipation. After you have those 3 you build up your points to your "36 point" tanking talent in your chosen tree. You will want a minimum of 50 points in 1 tree to pick up your "Expertise Talent".
Your base spec will look something like this. There are a bunch of filler talents you have to take to get down to the 50 point level, and are completely optional. These include Bloodworms, Hysteria, and Bloody Vengeance. Some people like to pick up Rune Tap instead - I have never been a big fan of it personally, but it all comes down to taste and personal choice. From there you should have ~9 or so points leftover that you have to decide what to do with. Most people currently stick them in Frost to pick up Lichborne.
My old spec used to look like this. Some people liked to use the last 3 points to pick up Morbidity to strengthen what is arguably Blood's biggest weakness in AoE threat.
In 3.1 Blood will change somewhat. The base spec will change to something like this. Note the filler talents again of Bloody Vengeance, Sudden Doom, Hysteria and Mark of Blood. Those points can be placed in other talents as you so choose. The end spec might look something like this as Blood is being encouraged to put more points into Unholy than Frost. Again, a lot of these talents are optional and highly dependent upon your playstyle.