When I started in professional computing in the early '80s, there were probably 30-40% women in areas where I worked. It isn't some kind of innate preference - that tired excuse that gets trotted out to justify any conservative position.
What's changed is this idea of "tracking", as soft as it is. It used to be that a philosophy major, like myself, or an English or history major would get a programming job because most schools did not even offer CS courses, much less CS majors. Now, there is this idea of a course of study along the lines of math and engineering that leads to a career with computers. It's not any one thing that accounts for this shift, but turning the decision to work with technology from a late-stage one to an earlier, long-term one probably doesn't help. It's now like a long corridor of slight but persistent bias - highly evident in the bitter, stupid comments in this discussion - that weeds out women from the field.