Skud linked to the "unlocking the clubhouse" book/study that CMU ran a few years back. There's an entire chapter in it titled :the nexus of confidence and interest". That's part of your answer - please go read it.
Sure, there will always be women who aren't interested. Just like there will always be men who are honestly just not interested in going into nursing. But at the moment, "interest" is so clouded by cultural expectations around gender roles - "computers are for dudes, nursing is for chicks" - that the idea of "interest" is effectively meaningless.
A more important point to focus on, that I think you're missing in this whole discussion - how is Open Source / Free Software <i>missing out</i> because women aren't participating? How can we change that, as a community?
Then go read the various link posted around this thread, where many suggestions regarding that have already been made :)