Many atheists, when trying to ground moral values, appeal to something like human well being as a guide for morality. This seems to be patently insufficient of a ground for morals, because it itself is simply a moral judgment. It is good to promote human well being and bad to stifle it is a moral value itself, and therefore can't be the ground of moral values. As I said to one commenter on Sam Harris' attempt to use this as a ground, "Saying that well-being of some sort is good is simply another moral claim, so it hasn't reached any sort of ontological base at all. If this is Harris' base, then he seems to have stopped short of a true ground for morality and settled for a branch. [In other words] you can't say that a moral value is itself the ground of moral value."
This is why most attempts at ethics today are silly little exercises in futility.