A rogue commenter reinvigorating the discussion at this post has led me to clarify something relevant to the abortion debate that I've been moving toward for a while now. There are a number of arguments on both sides of the abortion discussion that involve conceptual slips across important distinctions, and I think it's worth clarifying the assumptions that enable this process.
First, we need to separate out the following three-way distinction: biological humanity, what I'll call Warren-personhood, and moral status. Biological humanity is simply what biologists would classify as being human, as opposed to being a member of a different species or not being an organism at all. Warren-personhood is what most philosophers nowadays mean when they speak of personhood. I'm not convinced that this concept lines up with most people's notions of personhood, but it's become a technical term in philosophy for having certain capacities such as consciousness, self-awareness, the ability to plan for the future, and so on. I call it Warren-personhood because Mary Anne Warren's important pre-Roe article "On the Moral and Legal Status of Abortion" is the first instance I know of for this use of the term. Moral status is what it sounds like. Something has moral status if it would be wrong to treat it in certain ways for its own sake (and not just because it's someone's property or because it robs the world of beauty). Most people prefer to talk about moral status in terms of rights, but I prefer not to, because I think moral status is more expansive than rights, and I don't think rights are fundamental to begin with.
One more terminological matter is important before I say what I want to say. Here are two concepts: a three-sided planar figure and a three-angled planar figure. Those aren't the same concept. One concept has to do with how many sides the figure has, and the other has to do with how many angles it has. Philosophers will call these two concepts co-extensive. The extension of a term is the entirety of things that fall under it. The extension of 'tree' is simply all the trees. The extension of 'triangle' is all the triangles. The extension of each of these two concepts is the same as the extension of 'triangle'. The two concepts are co-extensive. Yet they aren't the same concept. Some concepts will be co-extensive but not necessarily so. It just happens that the concept "major party U.S. vice-presidential candidates through 2008 named Geraldine and Sarah" is co-extensive with the concept " major party U.S. vice-presidential candidates through 2008 who are women". But they might not have been if some other presidential candidate had selected a woman as V.P. or if McCain or Mondale had selected a different woman.
The pro-life position typically takes the first and last concepts in my list (biological humanity and moral status) to be coextensive, sometimes by means of taking the second (Warren-personhood) to be coextensive with each. But the pro-life argument doesn't need to rely on that. It can be done as long as moral status comes with being biologically human. One response to the pro-life position is simply to distinguish between these two concepts, as if that's the end of the discussion. But that response fails to consider the possibility that the concepts are distinguishable but co-extensive (or, more precisely, just that everything falling under the first concept falls under the third). All that would have to be true for that is that every biologically human organism has moral status. To assume otherwise is to beg the question against the pro-lifer by asserting without argument that human organisms might not all have moral status.
The only argument I've ever seen for such a position is to assume Warren-personhood is what matters for moral status, something the pro-lifer doesn't assume. Thus the argument assumes, in effect, what it's trying to establish, or at least part of what it's trying to establish, which is that Warren-personhood and moral status are co-extensive (or, more precisely, that nothing in the biologically human category has moral status unless it's a Warren-person). I'm really unsure that such a thing can be established without begging the question against the pro-life view. I'm actually pretty sure it can't, actually, or I probably would have seen such an argument, and I'm pretty familiar with the philosophical literature on abortion.
I'm not saying that this favors the pro-life argument very much. It;s more a recognition of why neither side is moved by the other. I've long seen the assumption behind this kind of pro-choice argument as question-begging, but I think this way of framing gets at my worry a lot more precisely. It's not really a matter of getting the concept of personhood wrong, as I've said in the past. It's a matter of two views on the relation between these different categories, with really little in the way of careful philosophical argument that either side can use to convince the other on its own terms of its stance on the foundational issue.