That's a very high quality thread over there - thanks to Peter Beattie for provoking such good discussion and to Ophelia Benson for hosting it. There are a couple of excellent commenters who are ably defending views similar to mine.
But I also find myself nodding along agreeing with the comments made by Paul W. in defence of The Moral Landscape. I don't think they go all the way to defending moral objectivity of the kind that error theorists deny, and nor do they defend a thoroughgoing consequentialism based on maximising something. But he doesn't claim that they do. He puts a very good case that the sorts of things that we usually have in mind when we talk about "well-being" really do lie at the roots of human moral codes. I haven't disagreed with that, but it's a point that I should, perhaps, emphasise more. Again, it doesn't get us all the way to maximising something - for example, it in no way shows that a society must prefer universal well-being in some sense to its own survival, if confronted with that choice. But there's still a lot of scope for convergence here if we can get the non-moral facts all agreed, and that's the strength of what Harris is saying.
I do disagree with the idea, which Harris pushes, that refusing to accept maximisation of well-being as the goal of morality is somehow akin to radical epistemological scepticism. I don't think this is a point where our spade is turned - i.e., where we hit bedrock. What I do think is that most of us value certain things that we can summarise as well-being, and that these things usually correlate with each other, although they can come apart in thought experiments and sometimes in real life. If you try to show that well-being really is just one of these things, or that they can all be reduced to one thing, you'll find that your view is contested, quite legitimately, but most of us can at least agree in a rough sort of way on what we want from laws, customs, policies, moral systems, etc.
It may not be a matter of maximising one of these things, so much as making a variety of judgments about moral systems as to how far they tend to meet them. There is room for legitimate disagreement about how "good" various moral systems are, but judgments like that don't have to be irrational, unreasonable, or arbitrary. They are constrained by the fact that we (meaning almost anyone likely to get involved in such conversations) really do have a lot of underlying shared values. Judged by those values, there will be plenty of clear-cut cases where certain moral systems have gone off the rails in one respect or another, or in many respects.
Obviously, I prefer this way of putting things to the way Harris puts them, but most of the book could be translated into this sort of language. It's mainly in the endnotes, where Harris deals with theoretical objections, that I think he'd need to make considerable changes before I could sign on. E.g., he has a note relating to Mackie that I think is quite wrong, and another note discussing Robert Nozick's "utility monsters" thought experiment where I think his theoretical commitments lead him to the wrong approach. But do read Paul W.'s long comments, because they say better than I could why there is a lot of good sense in what Harris is trying to do, and my arguments about his high theoretical positions don't take away from that.