I actually am trying something interesting, went 12/0/59 as unholy DPS spec (basically no tank abilities), and glyph dark death/scourge strike/ghoul. This is my DK dps spec and is supposed to be pretty good with 2h. I am going to multibox tank with this spec today a heroic or two and see how it works.

My second spec is frost tank spec, 12/54/5. glyph'd obliterate, rune strike, howling blast (last glyph cost me 300g, yikes). This will be my raid/tank spec, I did a quick OS10 pug last night and it worked ok - howling blast is nice at picking up aggro. Obviously the downside is that death grip is on longer c/d, and D&D doesn't pack as good a punch as unholy spec, and of course no ebon plaguebringer for extra 13% caster dmg --- but in raids, usually there will be a boomkin or another unholy DK to provide that buff anyways.

The problem always I had with unholy is that single target threat is fairly weak, while AOE threat is great. I figure I can AOE tank trash with the unholy DPS spec (wearing tank gear), and switch to frost spec when tanking bosses. I can multibox tank with the unholy DPS spec, and if I need to raid as DPS I can just put on my DPS gear and not have to switch any specs. I'm hoping to join a EoE pug as a DK dps tonight and see how unholy pure DPS spec does (I have good DPS gear, 4/5 t7 and around 4k ap).