I just found out that the freshman class at Caltech (my alma mater) is 37% female this year, the highest percentage ever. Apparently they've been heavily recruiting the women. My class, which entered in 1996, was about 25% female, and I remember that women were offered free transportation to prefrosh weekend while men had to pay their own way.
Caltech's administration is really bothered by the unequal sex ratio-- heck, a lot of students didn't like it much either, and the men tended to complain more than the women. :) As for me, I really liked my fellow students and never felt much affected by the fact that most of them were men. In fact, I remember feeling when I came to Caltech that I was finally at home. High school had more girls, but at Caltech the culture was hospitable to my interests and I felt like myself.
Other science and engineering colleges have a more equal sex ratio, but that's because they lower their standards for women. Caltech has never done that yet and I pray to God they won't bow to the pressure and start! Aside from the obvious injustice of discrimination on the basis of sex, it would just be a shame to create a student population in which the women are less qualified than the men. Caltech was hard and I sometimes felt like the dumbest student there (not an uncommon feeling, I think), so it was a great comfort to me to know that I had been judged by the same standard as everyone else and found capable.
May 2010 update: In 2009/2010, 38% of all Caltech undergrads and 30% of grad students are women, according to a Caltech newsletter I received. The 2009 freshman class is 42% female, which is the highest ever. After reading the article in that newsletter I'm no longer sure if Caltech still uses the same standards for both men and women, but I don't have time to blog it right now.
October 2010 update: The incoming freshman class is 40% female.