The Death Knight starting area will be an extended area in Northern Plaguelands. The developers feel strongly about keeping people interested in playing in the Old World so they are creating this new zone. I personally hate every zone that has the word "plague" associated with it. Admittedly, I didn't reach either of the current Plaguelands until after I'd upgraded to The Burning Crusade, which meant I really had no time for grinding there since Outland awaited alluringly. However, it will be interesting to see if they make changes to the existing Western and Eastern Plaguelands along with adding content to the North to make these areas more interesting to players -- be they Death Knights or not.