Thanks for the link - an interesting discussion, I thought.
You are right, I think: just because there are problems with the concept of personhood (as this pro-lifer argues) doesn't mean that we are naturally left with some sort of religious/pro-life position. On the contrary, we are simply left in a bigger quandry!
The main thing I want to react against here is the idea that by defining a concept we somehow absolve ourselves from having to make decisions of value - decisions about concepts often do involve value (as we agreed in the other thread). What people often do, therefore, is to use concepts (e.g. personhood) in a way that ignores this. It is as if they say to themselves, "If only I can prove that X is a person, then we would have a clear position on what to do about X" - but this is to beg the question? Why do we treat X in this way? To put it more concretely, if a foetus is a person, we must not abort it - but why? Because the possession of certain features entitles the foetus to certain rights? I think if we head down this road - as with animals and personhood - then we eventually end up in a mess (animals who are more persons than newborns, etc.).
My suggestion is that we stop trying to define concepts (in the hope that they will then make the hard decisions for us - whether X lives or dies), and simply admit that calling something one thing or another makes less actual difference than we think. Does it really matter how we classify a foetus? Either way - and I am not being pro-life here - whether to have an abortion is a horrible decision to make - and there are situations where this is a real decision. For instance, what if it was the mother's life versus the baby's? Whose life should we be 'pro' here? If we start weighing up the pros and cons, then we must really reject the absolutist version of the pro-life stance.
As for the religious perspective - whose religion? Whose interpretation of that religion? What about, "Thou shallt not kill", versus "Thou shallt not suffer a witch to live"? (Perhaps the last quote isn't an ideal example, but you get my drift - I've always taken it to refer to astrologers...).
The fact is, sometimes people kill, and sometimes people cause suffering. Sometimes, possibly, both of these are justified. Sometimes concepts can help us make these decisions, but - and this is my main point - the concepts shouldn't be assumed to make the decision for us (a decision which is hard, and calls into question our deepest values).
