The Lich King isn't particularly picky. If you're dead, he'll take you. And the Drae have been around about a year now, certainly enough time for some of them to make their way over to the Plaguelands and get killed fighting for the Argents, as the DK starter cinematic says.

Of course he knows! Velen is his pawn!
WE'RE ALL DOOMED!

Arthas isn't just randomly generating Death Knights out of nothing; he's raising the corpses of his opponents to serve him after death. Draenei death knights are particularly valiant draenei who fought against the Scourge and failed. Probably, when Arthas encountered his first draenei, his reaction was "wait, the Alliance and Horde are consorting with demons wholesale now?" - but heck, they fight as well as anyone else (better, in fact, thanks to the +hit buff).

As for the factions accepting them, Varian Wrynn and Thrall have formally allied with the Ebon Blade, so while there's probably still some lingering resentment, everyone's under orders to play nice.

David, can the next artical pleeeaase be on death knights? It looks like on wednesday me and a few members of my guild are rolling Deathknights on a Rp server, and although I'm confident with casual Rp, Deathknights are so deep. I have so many questions about how a DK will think, will she feel empty with out the Lich King's voice in her head? Will he or she be ashamed of what they are and go about insisting they're still paladins? What might Ner'zul have done to their memories? Resorecting dead memories so they can never forget that person they acsidently killed or whatnot. What is the genreal reaction to Death knights on Rp servers at the moment? Should I exspect rottern fruit? I've got so many ideas, and many more, I need some one to lay it all out on the table for me.