"Moral nihilists assert that morality does not inherently exist, and that any established moral values are abstractly contrived." - http://en.m.wikipedia.org/wiki/Nihilism
I have studied a bit of philosophy in my time. This type of nihilism seemed to me to be a "look how cool-cynical I am" position. It seemed lazy to me too; proponents don't have to prove anything, just demand that others prove competing theories correct.
That's how I feel about the subject in philosophical banter anyway. Moral nihilism as a guiding principle in real life is degenerate.
I suspect those who claim to believe such crap only believe it when they aren't on the receiving end of what they would consider degenerate behavior.