One of the biggest hurdles for women in politics, I suspect, is the unspoken fear (among male and female voters alike) that they'll be prisoners of their biology in ways that male politicians aren't. Historically, successful female leaders have resolved this problem in one of two ways - either by presenting themselves as desexual or post-sexual (see Elizabeth I, or Margaret Thatcher, or Condi Rice) or by presenting themselves as so commanding, so masterful, that sex and pregnancy and childbirth simply have no effect on their ability to govern (see Maria Theresa, who ruled an empire while bearing twenty children). For obvious reasons, the Elizabeth I model, in all its permutations, is far more common in modern political history than the Maria Theresa model - and this, in turn, explains at least some of the wild confusion and hysteria that's greeted the nomination of Sarah Palin. Nobody really knows how to respond to a prominent female politician who's actually a mother, rather than a celibate or a grandmother.
This dynamic, I suspect, may ultimately make the news of her daughter's pregnancy more damaging to her political prospects than it otherwise would be. The whole Maria Theresa model of female politics depends on projecting an air of command - a sense that yes, you're a mother, but you've got all that family stuff completely under control. Palin's "no sweat" approach to her recent pregnancy, while weird in certain respects, actually dovetails with this model: If we're going to elect a mother of five to high office, we probably want someone who can fly eight hours after her water breaks and be back on the job a few days after giving birth. Her daughter's pregnancy, though, raises the idea of sex and reproduction in a very different context - it presents female biology something that's wild and unpredictable and beyond the Palin family's control, and I suspect that's a bad association for a female politician to have.
This is especially the case because in modern America, out-of-control reproduction is something that's associated, to be blunt, with white-trash culture - with Jerry Springer and the trailer park. Sarah Palin has the potential to be perceived and portrayed as a working-class heroine - as an impressive working mother who has both her personal and professional spheres under control. But she and her family also have the potential to be perceived and portrayed as something straight out of reality television: As a sideshow act, rather than as role models for working America. That's the line that her candidacy is walking: We'll know soon which it's going to tip.