Originally Posted by Chicken

If you've read the leaked preview before, you'd note that the Death Knight does not replace your existing character. What I'm guessing will be the case is that you need to have done a quest line on one of your characters, and it then adds Death Knight as a class available to the races it's supposed to be available to on the character creation screen. This way you don't limit people of certain races/classes from accessing the class, while still limiting the races available to the class.