It's a problem: here you are, a master of your very own runed blade, a summoner of an army of undead, an immensely powerful tank and damage-dealer -- and yet no one will let you into the lore party over at Starshine Shadowstrider's house!
"Yuckie!" they say. "He got his powers from the Lich King! ...Eww!"
"But I turned against him!" you proclaim in your defense. "I wised up and came back to the Light, sorta. Okay not really but I'm still not completely bad! It's like... fighting..." your voice gradually gets quieter as party-goers stop listening, "... fighting fire with ... fire. Dammit. I'm gonna go kill Onyxia by myself! That'll show 'em."
Yes indeed. It's looking like the future's going to be pretty rough for Death Knights. As Nethaera explained, "It's most likely that Death Knights aren't going to find themselves being popular guests at the town picnic."
But Death Knights aren't alone in dealing with this sort of thing. "Keep in mind," Nethaera points out, "that Warlocks aren't generally welcomed with open arms either (depending on the culture of the race in question)." Here we have a good role model for plucky summoners of the undead: Warlocks are magic users who use the powers of the Burning Legion against the Burning Legion itself! Have not the Alliance and the Horde not gradually come to accept that just because some people happen to summon demons, cause agony, corruption and ruin, as well as steal people's souls -- that doesn't make them inherently evil, right?
So it is with Death Knights. So what if you betrayed all the peoples of Azeroth to go and serve the Lich King for a little while? So what if his taint of undeathiness is still flowing through you while you draw those unholy runes and create zombies? You can now go to Shattrath to have your picture taken with a Naaru! Your reputation should be squeaky clean.
While Blizzard may yet find a plausible reason for Death Knights to be fighting against the Lich King rather than with him, keep in mind that even if that doesn't work out, you've got one magic word that will make everything okay. Whenever people won't trust you or play with you just because it would seem that you might possibly be technically evil (on the outside)...
Just say: "Lore, lol."