ANALYSIS OF A DK TANKING BUILD

I think a DK with any spec can tank any normal instance perhaps even heroic instances with some good tanking gear set, some different runes and frost presence.

+ My point of view
If you want to get the best out of your DK tank for raiding, I think you need to focus on your niche: spellcaster tanking, but not at the cost of your melee tanking viability. Acclimation (frost40) and Spell Deflection (blood15) are the key. Since DKs rely on a high parry chance for tanking, I bet Spell Deflection will mitigate a significant amount of spell damage and for regular tanking, the Forceful Deflection (b5) talent is very helpful too.

I came up with 18/53/0
http://wotlkwiki.info/talent/deathknight.html?b=623c51403q5305501303013031013013231051zd

Not only does it make perfect sense that since frost presence is for tanking, the frost tree is also for tanking, but everything just seems to fit without an abundance of dps talents.

+ Talents in the Frost Tree
Icy Touch is a core tanking ability, because it slows down attack speed (similar to thunder clap) as well as casting speed (fitting the DK niche) by 15%. Mastery Attack speed is further reduced by Frigid Dreadplate. Bringing the Icy Touch CD down from 6s to 1s (Imp frost0) seems neat. I chose not to add a 15% chance to freeze to Icy Touch (Frostbite f0), because it's randomness may seriously hurt you in mob positioning more than it may benefit you in the form of Frost Rune Mastery (f25). Furthermore, bosses are immune to the freeze anyway.
Many frost talents go without saying: armor, spell damage, +hit, parry, +4% hp for the party.
On the way we pick up some neat other talents: Frozen Rune Weapon (f10), extra frost damage and applies frost vulnerability. With Chilblains (f25) we add a slow to Icy Touch and Mind Freeze. Aneurysm (f35) allows you to use Mind Freeze for free either to interupt vs casters or simply free damage and threat vs melee. Frost Strike (f40) gives a on-next-melee ability similar to heroic strike.

+ AoE Tanking
Three other new abilities can be used as an initial combo for AoE tanking: Deathchill (f20) 100% crits for 6s -> Hungering Cold (f50) AoE silence + freeze -> Howling Blast (f30) AoE damage, triple damage vs frozen targets. That's some neat base threat: triple critical damage to all targets multiplied by 110% because of Tundra Stalker (f45). After 30s a mage can Frost Nova to get another triple damage Howling Blast. Further AoE threat is applied by spreading diseases with Pestilence and erupting them with Blood Boil, which also acts as an AoE taunt. Death and Decay, with its CD reduced to 30s can be used, but take care as mobs can be feared by it.

+ Other talents to consider
So far on what this build gives you, because it misses out on some good stuff: Veteran of the Third War (blood30), +6% total str, +3% total sta. It is possible to get this talent and Frost Aura (f35) +4% hp party. In the unholy tree, we miss out on most importantly: Bone Armor (u25), Magic Suppression (u25) and Anti-Magic Zone (u30). You cannot get these talents however without sacrificing both the parry talents in blood5/15 and Acclimation (f40). It's giving in one for the other, while it requires you to pick up a lot of mediocre talents for tanking in the unholy tree and lose significantly in the frost and blood tree. An example of hybrid tanking in an unholy spec if you ask me.
One more subject I'd like to touch is Death Grip. As far as I can see, it looks like it works more like Mocking Blow than like Taunt as well as giving the DK a neat way to pull. If it turns it is a Taunt however, Unholy Command (u5) is definitely worth it, considering the long CD.

+ Conclusion
The Frost tree seems to fit the DK tank best in terms of general tanking utility. Some parry talents in the Blood tree can be picked up while sacrificing hardly anything in the Frost tree to give a 18/53/0 build for level80. This gives the DK a good baseline for tanking and makes him superior for mitigating magic damage and tanking trash packs of multiple casters.
There are some useful tanking talents in the other trees, designed by Blizzard to give each tree tanking viability, but these do not seem worth it to go that deep into the Blood or Unholy trees.

This concludes my analysis of DK talents for tanking based on the talents and abilities at http://wotlkwiki.info
I hope you enjoyed the read. I can't wait to find out more about DK tanking, so share your thoughts!