We've been seeing several Death Knights running around and it seems like, in my experience, they aren't afraid to tank whenever you need one. In fact it seems like the presence of Death Knights have really helped the tank shortage problem as was intended. Healers seem to be harder to find now than tanks, at least in my experience leveling up.
Being a new class, it still has some issues that need to be addressed. Ghostcrawler floated around some ideas about Death Knight tanking in a post he made in the official forums. He mentions that they are not prepared to announce specific changes yet so don't be disappointed if some of the thoughts don't get implemented. His thoughts are summarized below. Click here to see the actual post.
1. Boost frost presence mitigation, lower mitigation of other abilities. DK's tank well with cooldowns and don't do as well without them.
2. Icebound fortitude could see mitigation scale based on defense.
3. Expect buffs for Blood tree tanking talents
4. Rune strike might get decreased damage and increased threat
5. Death and Decay is not supposed to be a tool used to gain runic power out of combat.
6. Horn of Winter and Unholy blight could get decreased runic power costs.
7. The Shadow of Death talent isn't intended to be used for escaping durability damage or resurrecting by zoning into an instance.
8. A new spell might be added to let you raise fallen allies (wait... they're getting a battle rez? 0.o) so you don't have to choose between bringing back a player or your ghoul.
Although I'm not really very familiar with Death Knights much less Death Knight tanking since I don't play mine much yet, it seems like the ideas being floated around look interesting. Being a new class, it's acceptable that Death Knights still have some issues but it's good to see an effort to make things better.