I find Matt Yglesias' case that Ron Paul isn't really a libertarian quite compelling, but his follow-up contains a mistaken argument about libertarianism and abortion:

Some people want to tell me that if you accept the erroneous metaphysics of the anti-abortion movement, that then treating women who terminate pregnancies as criminals makes perfect libertarian sense. For one thing, I don’t accept the erroneous metaphysics of the anti-abortion movement. But even if you do, this doesn’t make sense. The “pro-life” position amounts to a conjunction of the proposition that a fetus is a moral person and that a pregnant woman has a strong legally enforceable rescue duty. But Paul doesn’t believe the state should tax people to feed the poor, or impose rescue duties in any other context. Rather, he simply seems to feel that pregnant women aren’t really people.

First, I agree that the "fetus is a person" position rests on some demonstrably weak metaphysical grounds. But Matt's second argument against anti-choice libertarians, which attempts to move internally from said weak metaphysical premises, doesn't fly. Though he's a little unclear about what "rescue duties" are, I understand his argument to be something like "carrying a child is an extraordinary burden on a woman, so legally requiring her to continue the pregnancy constitutes requiring she take significant effort at high cost to herself to take care of a vulnerable person. Since libertarians don't generally believe in imposing duties on individuals to take care of the vulnerable using state power, they can't consistently impose such a duty in the case of abortion."

But the fetus isn't just "x random vulnerable person" if we're accepting the metaphysical premise; it's the woman's child. If that's the case, the proper analogy isn't "taxing people to feed the poor," it's laws against child neglect. Only in the latter case is the parent compelled to take significant action to ensure their child, as opposed to a random vulnerable person, is properly cared for.