Lately, when you load up WoW the news feed should have a link to the new WotlK stuff that has come via interview/press release etc.
It said:
"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."
Hope that helps.