In 3.1, the next round of class changes have gone up and Death Knight certainly got a blurb of their own. Probably the changes are slightly less sweeping than some other class but interesting all the same. Death knights are a bit high on DPS and encourage more people to spec Blood and Frost over the overwhelmingly popular Unholy. It is sure that a couple of nice potential buffs are given to Death Knight Tanks. Let's look at them one by one.
   Pestilence no longer causes damage but just spreads diseases. Blood Boil is intended to be the general area attack, and has been changed to be castable on targets with no diseases on them, but does extra damage if diseases are present.
Being able to both spread our diseases and cause massive damage was probably a bit much. In the end, it may be a slight nerf to AE damage, but not by much. Blood Boil will pick up most of the slack, and we'll just have use a couple extra cooldowns and wait for our blood runes to refresh before doing massive damage.
Unbreakable Armor absorbs a flat amount of damage that increases as your armor increases. It no longer boosts armor. Depending on the amount of absorption it grant, this will be nice because it means magic damage absorption as well as physical absorption and could make it a valid button to hit for a DPS DK taking environmental damage or even a PVP DK being focus fired. It's difficult to say too much more without solid numbers, but on the whole this very likely buff to Frost DKs in general and definitely to Frost Tanks.
The Frost tree has been shuffled. Among other things, PvP talents such as Endless Winter are closer to the top of the tree where Blood and Unholy Death Knights can access them.
Blizzard has said that they want to let all trees participate in PvP among all classes, so while putting things like Endless Winter and Chillblains lower on the Frost tree might do that. On the PvE side, Glacier Rot moves further down the tree. That skill mostly buffs deep frost talents and yet it's nearly unavoidable if you're a Blood or Unholy DK trying to get the first tier or two of frost.
Sudden Doom procs a Death Coil rather than requiring an additional button click. It works similarly to Shamans' Lightning Overload. To free up your attention, this is a nice change so you can focus on optimizing your rune spending without wasting a global cool-down on a random Death Coil. It's a positive change that should help Blood Death Knight DPSers keep their rotations tight and efficient.
Magic Suppression and Blood of the North have been reduced to 3 ranks for the same benefit. It is always a bit of a struggle for Blood and Unholy tanks to get Magic Suppression or Blood of the North, which means that it gives up access to Lichborne or Improved Icy Touch in Frost or useful but not absolutely mandatory talents like Bladed Armor or Wandering Plague.
At present Blood Gorged grants armor penetration instead of expertise. Maybe this is the most perplexing change. To put Armor Penetration lightly, it sucks. While Blood Death Knights do the most physical damage of the Death Knight trees on average, Armor Penetration is still not exactly desirable anymore. However, it may be that Armor Penetration is due for overhaul that will make this change look better with the changes to Sunder Armor and Faerie Fire already announced. It's possible that Tier 8 Death Knight gear will have so much expertise on it, they're afraid of Blood DPS Death Knights hitting the expertise dodge cap too easily. We'll have to sit tight on this one before we can make a better judgment.
Remember that those changes we above-mentioned are not comprehensive and final, we will update this sooner or later.