As we've seen from the exit polls, she pulled ahead based on strong support from women.
UPDATE: I should say we're seeing some talk of a "Wilder effect" possibly doing Obama in. I don't buy that. If you look at the breakdown of the results, you'd need to believe that white women, but not white men, are inclined to lie to pollsters about that. More likely we're looking at a combination of gender backlash, plus the fact that Obama was so widely perceived as likely to win led independents to vote for John McCain in the GOP primary.