What kind of rotation should death knight use while tanking?

Obviously this is going to depend on what spec you are and what talents you took, but otherwise they’re generally straightforward. Frost is usually going to want to open with Icy Touch, and Unholy with Plague Strike - This is so that Tundra Stalker / Rage of Rivendare can kick in for your subsquent disease. Once you get both diseases on a target, what you do next is determined by what you are tanking - One mob or several.

If you’re tanking several, Pestilence should be your very next ability, and often you will follow this up with a Blood Boil, then your FU ability. Depending on your own preference, you may also want to ignore the Blood Boil and use your remaining BFU runes to drop Death and Decay. Opening with D&D at your feet and pulling a group into it is also common practice.

The guildies I run dungeons with the most regularly are usually a Mage, Warrior and/or Elemental Shaman, and they love to throw AoE all over the place and power through the trash. To work with this as Frost, one of my favorite opening combinations is as follows:

If Rime does not proc:
D&D -> IT -> Pest -> Blood Tap -> Deathchill (if it’s up) -> Howling Blast

If Rime does proc:
D&D -> IT -> PS -> Pest -> Deathchill (if it’s up) -> Howling Blast

Right now, my Howling Blast will crit for 4-5k on targets with Frost Fever. Combined with Death and Decay, this rotation frontloads an enormous amount of threat. Continuing throughout the rest of the pull, I maintain a priority rather than a strict rotation:

#1 Frost Fever must remain active on every target I am tanking
#2 Use Howling Blast whenever the cooldown is up

Between those two priorities, keeping up Blood Plague is also important, but not as much as Frost Fever, as the damage it does is rather insignificant compared to that of Howling Blast. For the moment, I also make liberal use of Pestilence while my Blood Runes are up, now that the cooldown has been removed. This will change once the damage component is taken off in 3.1, however.

Unholy will want to conserve their Runic Power so that they can get Unholy Blight up as soon as possible, and an Unholy DK’s Blood Boil is quite a bit stronger than Frost’s. They also, as I understand it, like to make liberal use of Death and Decay.

Tanking single targets is slightly different. There’s no reason to use Pestilence on just one mob, and both Frost and Unholy have talents that convert Blood Strike Runes into Death Runes. Load up your diseases as normal, and blow your first two Blood Runes on Blood Strikes. This is more beneficial for Frost than it is for Unholy, but it is important nonetheless. It gives you the opportunity to unload three FU attacks in your next rotation for a very powerful threat jump. Frost Obliterates and Unholy Scourge Strikes both hit very hard, and three in a row is a beautiful thing.

At that point, your priority will be to maintain both diseases on your target and fill the gap with as much FU ability usage as possible. Don’t forget to use your RP abilities, and pay attention to your Rune Strike hotkey - This is a very important ability for maintaining threat. Many people recommend making macros for each of your abilities that attempt to use Rune Strike whenever you press them to simplify the process. I disagree with this suggestion because A) It is rather time consuming and more importantly, B) It doesn’t give you the option not to use Rune Strike if you need the RP for something else, such as AMS or IBF.