The Celtic Chimp offers an excellent rebuttal to Thomas Metcalf's argument that objective moral facts exist. I suppose Metcalf himself cannot be blamed for throwing together an equivocation fallacy and a laundry-list of moral straw men — he's merely repeating faulty arguments well-established in the philosophical canon — but one cannot admire Metcalf's credulity or the shallowness of his analysis.
I have only a few quibbles with the Chimp's well-argued rebuttal.
First of all, it's necessary to distinguish between facts and truths: facts are truths that are uncontroversial (usually by virtue of being directly (or shallowly) epistemically available): the facts are what we observe directly, and can quickly confirm that everyone else observes as well. In this sense, we cannot be in very much disagreement about the facts: If we are in disagreement about some proposition, it is not a fact (although it might well still be true).
There are, of course, matters of truth on which a lot of people disagree; for every matter of truth on which a lot of people disagree, however, there is some body of fact to which (most) everyone does agree. We may disagree about who shot JFK (even though we do agree that someone did so; we have good reason to believe there is some truth to the matter, even if we don't know the specifics.) But there are matters of fact on which we all agree: The Zapruder film, the testimony of witnesses, are all are facts, in the sense that the images on the film are what they are, and the witnesses said what they said.
Epistemic reasoning in general tries to reason from the facts to non-fact truths. It is a fact that when I dropped my coffee cup a moment ago, it fell to the floor, and it is a body of fact that every time I've dropped something, it's fallen to the floor. From those facts I employ the scientific method to conclude that the simplest explanation for those facts is that there's some "force" (let's call it "gravity") that causes the things I drop to fall to the floor.
If you want to dig to the phenomenological , the facts are that I saw the coffee cup fall to the floor, and that I've always seen things fall to the floor. In which case, the simplest explanation is that they really did fall to the floor; the alternative explanation that I might have a consistent delusion about things falling (or things in general) is more complicated.
We see the result of this equivocation fallacy in Metcalf's stunningly retarded non sequitur, "If [moral intuitions are evidence], then objective moral realism obviously wins."
Moral intuitions are evidence, because anyone can observe that person A has intuition X (you just ask them). However, just because intuitions are evidentiary doesn't mean by itself that objective moral realism is the simplest explanation for this evidence.
Metcalf is correct that the concept of objective moral realism doesn't pose a problem for atheism any more than objective physical reality poses a problem for atheism. However, Metcalf's mode of reasoning is a problem not just for atheism but for skepticism and scientific reasoning. Metcalf equivocates fact and truth, asserts the lack of an absolute disproof, enumerates a few competing straw-men, and calls this not only epistemic justification, but obvious epistemic justification. It does not take a Wittgenstein to see that this is precisely the same sort of "reasoning" that theists, woo-woo's and credulous idiots in general regularly swallow.