One of the more interesting discoveries I made at Cato Univeristy is that although most of the older libertarians there seemed to be strongly pro-choice, the majority of the younger libertarians were pro-life. I'm sure there's a selection factor at work, since the older attendees paid their own way to the conference and the youngsters were often there on scholarship (like me), but it was still an interesting dynamic.
For a libertarian (and, I'd argue, for everyone) the abortion question rests solely on one issue: is a fetus a human being? If so, then a libertarian must seek to protect that life on the same terms as any other. If not, then there's no reason for government to get involved with abortion at all.