Unholy: This 0/10/61 DPS build is one of the top competitors for DPS and the one I'm personally sporting. While it may seem like sacrilige not to head into the Blood tree, the Unholy tree does enough shadow damage that Black Ice's new shadow damage multiplier wins out. In addition, having 130 runic power comes in handy when you're trying to unleash a Gargoyle and still have enough leftover for Unholy Blight and a couple Death Coils. In this build, you get Frost Fever and Blood Plague up, then keep the Scourge and Blood Strikes coming for as long as your glyphs keep refreshing the diseases, adding your runic power dumps as needful. One note of caution on this build: You'll be flying without Subversion, so you'll want to make sure you're aware of the extra threat.
Blood: Blood was the redheaded stepchild of the Death Knight trees in 3.0 as far as imagined effectiveness went, but that really shouldn't be true in 3.1. Blood easily keeps pace with Unholy in the DPS department. Here's one of the more popular Blood/Unholy builds. This build focuses a good amount on heavy-hitting physical damage, so armor penetration becomes surprisingly useful in this build. Army of the Dead in the Unholy tree makes the disposable Ghoul a little more effective. The rotation should be more or less like Unholy, but use Death Strike for your Frost/Unholy rune strie, replace Gargoyle with Dancing Rune Weapon, and replace Blood Strike with Heart Strike. If you're afraid of losing AE potential with this build, remember that Blood Boil can hit undiseased targets now, and Blood talents have some pretty hefty buffs to that ability.
Frost: Frost, especially 2-handed Frost, continues to be somewhat of the dark horse DPS build. Not many Death Knights consider it, but it still manages to do very respectable DPS. 2-handed Frost DPS sometimes seems to me like it could be called the class' best kept secret. This build is one the favorites for solid 2-handed Frost DPS. You'll actually focus on churning out Obliterates with this build. Save the Howling Blasts for AE DPS or Rime procs - which you'll have a decent amount of, since you'll be using obliterate as fast as you can churn out the runes for it. You'll be using Frost Strike for your rune dumps, of course. This build also includes Improved Icy Talons for a little grouping love.
Frost: Frost remains the stereotypical tank build, as it seems a lot of people failed to get the message that Blizzard abandoned the Frost is tank thing back before the Wrath Beta. That said, it's definitely a more than passable tank tree. Here's a Frost Build to try out when tanking. Howling Blast is here to help with AE threat grabbing, and it goes into the Unholy far enough to grab some extra help for Death and Decay. It's worth noting that a lot of people like grabbing Bladed Armor for extra threat in this build. If you do this, it'll probably best to take the points out of 2-handed weapon specialization and Acclimation.
Unholy: I'm still an Unholy fiend myself, so this is the tank build I'm using too. Even with repeated nerfs to Bone Shield, Unholy remains a very respectable tank tree. Try this build on for size. It focuses primarily on picking up all the delicious magic absorption and threat goodies in the Unholy Tree, leaving Blood and Frost for the first tier defensive abilities. Improved Icy Touch counts here too, the extra slow means fewer hits against you. Like Frost above, a lot of people like to take Bladed Armor for the extra threat. If you decide to go that route, Taking points out of Virulence and reducing Wandering Plague to 1 point is probably your best bet.
Blood: Yes, the Blood Tree can tank. For a while, a lot of Death Knights swore by a Blood build to tank 3-drake Sartharion. But even though Sarth is yesterday's news, you can still get some very respectable mileage from Blood Tanking. This build gives you the all of basic defensive cooldowns and abilities of the Blood Tree, a lot of strong physical-based single target threat, and some AE threat in the form of Heart Strike, Bloody Strikes, and Morbidity.
But where is Dual Wielding?
Dual Wielding is still sort of a playstyle in flux right now, but some Death Knights are still plugging along looking for ways to use it in 3.1. This thread on Elitist Jerks suggests a few ideas, but most of them assume you have some pretty sweet raid gear and are on track to get more.
Myself, I'd wait for 3.2 to try dual wielding again, once we see what the devs have in store. Some people are claiming it should be as simple as converting weapon strikes to hit with both weapons. I'm pretty sure this would be the exact wrong way to go about it, since it would immediately remove the major reason 2-handers have even been able to stay head of dual wielding in the first place. Without the weapon strike edge, Death Knight 2-handers stand a very good chance of going the way of Enhancement Shaman 2-handers.
But anyhow, 3.2 is a ways away. For now, take any combination of the builds I have linked above, go forth and conquer. New class or nerfed class or whatever, Death Knights are still plenty enjoyable in 3.1, and they're even more enjoyable with a good build.