Anyone interested in why there are so few women in computing and how that can be changed must read Unlocking the Clubhouse. It summarizes Jane Margolis and Allan Fisher's research during 1995-1999 on why women made up only 7% of the first year students in computer science at CMU, and why they dropped out at double the rate of men. It provides a readable summary of academic research in this area, from infancy and childhood toys to the high school and university experience, as well as the results of Margolis's own study. Perhaps the most depressing part is that the book was published in 2002, but its descriptions of the "culture of computing" are still familiar, 12 years later. It made me realize that during my undergrad years, there is a good chance that I was involved in creating some of the "men's locker room" culture the book describes. (I think I have become more socially aware as I've gotten older, so I hope I don't do that now; If I do/have, please let me know.) However, the book also shows that change is possible, and doesn't need to take decades: female enrollment went from 7% in 1995 to 42% in 2000, in large part due to changes that were put in place during this study.
This is a subject that has been actively discussed on the Internet in the past month, since The Information interviewed Paul Graham of Y-Combinator, and discussed discrimination against women (Valleywag follow-up, Paul Graham's response). Co-incidentally, I've been talking to many of my female friends in computing, since I attended a Google Ventures event that had pretty close to zero women. I read this book because it was frequently recommended.
I'm not sure I have anything to add to this discussion that others have not already said better than I could. However, I would like to point out that women in startups and venture capital are even more poorly represented than in computer science programs or in the software engineering world as a whole. The startup world is all men, pitching ideas to other men. Many businesses that are targeted at women are started entirely by men. I think this happens for many of the same reasons women don't apply to CS programs, just shifted 5-15 years later in life. Similarly, the situation for blacks and Hispanics is also dire, and for many of the same reasons. I'm hopeful that I will see this change in my lifetime, and I'm going to try and help however I can.