Go read Kirrily Robert’s OSCON presentation on women in open source. What I find interesting beyond the basic gender issues is a cultural problem often found in software development communities — a general lack of hospitality for newbies. As the article points out, when we shut people out (either unintentionally or intentionally), what we’re really doing is limiting the scope of what we can achieve.
And we should expect to have to go to extraordinarily lengths to fix this problem, because lack of diversity is a self-reinforcing problem. Non-diverse communities generally become more and more out of touch with the things they need to do if their goal is to achieve diversity. All sorts of habits that are invisible to members but incredibly off-putting to the people who are excluded creep in. Breaking that pattern is tough, but worth it.