I'm pretty sure that Bliz has stated you only need a lvl 55 character to create a DK.
I just read the WotLK FAQ, and that's correct.

Wow. That's been really dumbed down.

Last time I read anything about it, the plan was that tou need to get to level EIGHTY with at least one character, then embark upon a quest "similar in difficulty to the Warlock epic mount quest", which will then let you create a Death Knight character who starts at level 55.