A few days ago, Instapundit Glenn Reynolds blogged this pro-choice argument. He has since updated it. If you're following the comments on this post or some of the comments on The Corner (and keep scrolling), you'll see that Reynolds doesn't include the strongest objections to his argument in the update to his post...even though one might say that it appears that he has received constructive notice of them.
The argument revolves around Instapundit's observation that there is no duty in law to rescue a drowning infant one happens upon in the street.
One counterargument to Glenn's position is to say that the relationship between a mother and an infant isn't analogous to the relationship between a stranger and a drowning infant one happens upon. Under the law, parents are (at a minimum) obligated to prevent their own infants from drowning, should they happen to observe that their infant is drowning.
Another observation that has been made about Glenn's argument is that it only speaks to what is legal, not to what is moral. It's not clear that Glenn is trying to address the morality of abortion in his post, but only the question of whether it ought to be legal, even if it is thought to be significantly immoral, so I don't know how much force this objection really has.
The strongest objection to Glenn's position is the point that The Raving Atheist brought to his attention. This is the objection that there's a big moral difference between allowing a drowning infant to die by refusing to intervene and actively taking steps to end the life of the infant. In abortion, at least as it is currently practiced, the life of the child is actively terminated. Presumably, Glenn does not think it would be morally acceptable to end the life of the drowning infant in order to avoid having to rescue it. At any rate, the laws he refers to as the basis for his argument certainly allow no such thing.