Interestingly enough, one of Michael Mahoney's papers, "Boys' Toys and Women's Work: Feminism Engages Software", weighs in on the topic. He asks why it was that women were synonymous with computers in the Grace Hopper days, but the field evolved not only to be male-dominated, but "masculinized".
I don't have much to offer in the way of suggestions for how to increase and retain women in IT. Studies by people such as Jane Margolis of UCLA reveal that outside of the US, many countries have nearly equal female and male participation in the computer field. JM's research includes testimonies from women from Asia who decided to stick it out in computers because it was economically liberating. Although they arguably face ostracism, discrimination, etc. as US women do, they remain in the field. I think that part of the reason US women don't choose (or stay in) IT is because there are many other financially lucrative options available to them.