Cameo of Dethecus has a good idea -- if there's a specific name for your Wotlk Death Knight that you want, probably best to grab it right now. You can create a character with the name on your server right now, and then when WOW Wrath comes out this week, delete the placeholder character and the name should be freed up for your DK.
During the Wotlk beta, I called my Death Knight Wotlk Dingbat, only because it was the first thing that jumped to mind. But it was a little silly to hear the great Wotlk knights of the Scourge ordering me around as Dingbat, so I'll probably go with something a little more threatening for the actual release: Marton? Erathor? Killahdk?
Whatever it is, now's probably a good time to check and make sure your name is free and get a toon in place before someone else takes it. And please: spare us from the Wotlk %26quot;Darth%26quot; names. Nobody, including Arthas, wants those.