So Goblins and Worgen are going to have death knights like all the other races currently in World of Warcraft. However here is the rub I came across today. The Death Knight starting area is currently set right before Wrath. That would mean all know Goblin and Worgen Death Knights would also enter the Warcraft time line as well. So these Death Knights should already with with the Alliance and the Horde. Where the other Goblins and Worgen will enter the time line after Wrath. This would make the motives of the Death Knights very different from the non Death Knights.
A Goblin Death Knight might have come before Thrall after the scourge invasion looking for vengeance on the side of the horde knowing of few others that would take him.
My hope is that Goblin and Worgen get there own Death knight tales and aren't just dropped into the Death Knight starting area like they were always there. wink wink. Or During the battle for Icecrown the Goblin and Worgen Death Knights are found and freed. there are many options
Hearthcast Episode #264: An Closer Look At The WoW Token - Hearthcast Episode #264: An Closer Look At The WoW Token - Rewt, Freckleface and Dedguy talk all about the WoW Token!