Blizzard have updated the official World of Warcraft Death Knight page with a lot of sexy goodness.
Death Knight Class Information
The fearsome death knight, World of Warcraft’s first Hero class, is not your average adventurer seeking to prove his or her worth on Azeroth’s fields of battle. No longer servants of the Lich King, they begin their new calling as experienced, formidable adversaries, heavily armed and armored and possessing an arsenal of deadly and forbidden magic learned in the Lich King’s thrall. Here is an introduction, subject to change, of some of the death knight’s core gameplay mechanics and abilities: Unlocking and Creating the Death Knight
Simply have a character of at least level 55 on the World of Warcraft account you play, and you will be able to create a new level-55 death knight of any race (if on a PvP realm, the death knight must be the same faction as your existing character). Upon entering the world, your neophyte death knight will undertake a series of quests designed to teach you your new abilities. You will be able to create one death knight per realm, per account.