I don't think there is a libertarian position on abortion. There was a study done by a graduate student at UCLA that found that about two-thirds of people you would identify as libertarian are pro-choice. From a philosophical perspective, libertarians generally believe the appropriate role of government is to protect life, liberty, and property. The question is, is forbidding abortion a way of protecting life, or should it be viewed as a restriction of liberty? There's a plausible libertarian case on both sides. People who are consciously libertarian are more respectful of the other position on abortion, in my experience, than most pro-lifers and pro-choicers. I do not think there is an official position.From an interesting Atlantic interview — via Instapundit — titled "America's Libertarian Moment" ("A longtime libertarian policy wonk talks about whether the philosophy can save the GOP -- and why he still doesn't think Rand Paul can win the presidency"). Via Instapundit, who says: "Rand Paul doesn’t have to be elected President to change the direction of the country."
I like libertarians when they back off from stark ideology, which really is not presentable to the American people, who require a closer approximation to something that feels like humanity. It's fascinating to watch Rand Paul supply the performance in the role of Libertarian that Americans can watch.