A summary from YodasEvilTwin on Slashdot:
"The internet is dominated by sexist men, which discourages women from getting involved in related fields."
I add a bunch more caveats, references and empirical data, but that is a good summary of how I interpret the evidence.
There is currently a responsibility-dodging contest between industry and academia over who is to blame for the declining enrollment of women in Computer Science and declining employment of women in software development. I hear people in industry bemoan the "empty pipeline", while academics maintain that women aren't entering their programs because of perceptions of the industry. I have compiled some data that may help resolve the question by highlighting a third factor common to both: access to an Internet-based culture of computing.