But will it really ruin the lore? Daniel already pointed out very insightfully that Gnomes definitely have a dark side -- we all seem to overlook the fact that Gnomes can be Warlocks, and that doesn't make the Felguard they send after you any less scary. And while yes, on launch day, there'll be a lot of Gnome DKs running around (the starting areas will be chaos no matter what gets rolled), after that it should settle down for sure. After all, if Gnomes were all people wanted to play, we'd never see Humans or Dwarves on the realms, and there are plenty of those.
So I'm all for Gnome Death Knights, pigtails or otherwise. If nothing else, it'll give us Horde something to laugh at -- right before we go in for the kill.