The National Center for Women and Information Technology has produced a great report called Women in IT: the facts. The summary slides set out a crystal clear business case for boosting the number of women in tech.
What's clear is that you can't just "decide to employ more women". That won't work. The whole problem is there are no women to employ - they're all busy feeling alienated and going into PR.
This report shows just how important it is to take a big-picture view of the issue. Going into schools to speak to girls about why technology is fun is a better plan than trying to increase the number of women on your team, and getting downhearted if and when it doesn't happen. It's taken a long time for us to slide into a position of there being so few women interested in tech and science, and it's going to take a long time to climb back out.
The report is US focussed, so the numbers aren't accurate for UK or Europe. But the pattern and the issues are the same across these countries, so it makes interesting reading. I found it through the Girls in Tech blog.