The comment above you (well right now) indicates that women are less experienced coming into introductory classes. It makes sense that they'll have a higher failure rate, as it's difficult to compete against people who know more than you.
In my experience, they're not only less experienced, but they're continuously falling further behind in experience, at least for the first couple of years. They try to approach the first couple of years of CS the same way you treat the first couple of years of any other major, like math or biology: read the assigned reading, master the concepts, do the homework, and study for the tests. Take a lot of other interesting courses, and spend the first year (or two) deciding whether to major in CS or History or maybe Religion with a pre-law emphasis. Then, as a junior, start thinking about a substantial senior project that will introduce you to practical work. Just like any other college kid.
But that just doesn't work in CS, not at most schools. The curriculum is designed for the average (or slightly below average) student, who already has some programming experience and OS knowledge and who is actively engaged in acquiring more experience and more savoir faire. An intelligent person with no experience can catch up, but only by focusing hard on CS and putting in a lot of extra work to catch up with the other students.
Is there any point in accommodating anyone else besides the lifelong hobby hackers and the focused enthusiasts? I think this is an important question. From a competitive standpoint, there's clearly no point. There are plenty of hard-core geeks, and it seems intuitively obvious that the curriculum can be more advanced and interesting if you take the students' strengths for granted. Washing out everyone else means you can produce graduates who are more deeply knowledgeable about computing and who are better equipped to be productive in typical industry jobs.
On the other hand, it homogenizes the field and bakes in the personal peculiarities -- positive and negative -- of the kinds of people who get deeply interested in computers as teenagers.
Obviously the PC answer (as well as the "big picture, good of society" answer) is to accommodate as wide a range of people as possible. At selective universities that is probably feasible, and if feasible, is the right answer. However, I got my degree in a non-CS field and only took CS classes when I was unemployed for a while early in my career. I took those classes at a community college and at a night extension of a crappy state school. In those classes, it was clear that some of the the geek/hobbyist students had a chance of being decently productive at industry jobs, but the students who had not done any hobby programming, whose only knowledge came from the curriculum, were completely hopeless. It was obvious they would never be competent enough to contribute in any industry job unless their primary competence was something other than computer science. For vocational training, I think it's unrealistic to encourage these "lightweights" to pursue a degree in computer science, because they'll never achieve a useful level of competence. You might as well wash them out and focus on the lifelong geeks, who at least can become competent coders or QA guys.
This is exactly the reason why most folks will never program; not because they don't have the capability to be competent, but because we have this odd culture around programming. There's this belief that CS is something that you have to start when you're young and be obsessed with your whole life to even do.
In any field- math, bio, art, music, the people who are best at it are those types- the start-early would-do-it-for-free types. But there's a whole lot of people milling around who could be second best. In other fields, being second best or not that into it is acceptable.
In computer science it's not. And more importantly, because programming and computer science are often equated, this prevents a whole lot of people from programming.
And who needs to program? I'd argue that the time is past where only computer scientists need to program. Everyone needs to program; biologists, mathematicians, linguists, social scientists... (maybe not the English majors, but w/e) and this idea that programming can only be done by people that started when they were 5 year old boys is a huge loss.
