I noticed a down vote on Do open source projects have better or worse female participation than proprietary projects?. As my comments there show, I think this question is both important and answerable with evidence:
Although it may be (perhaps counter-intuitively) easier to measure this for closed source projects (by looking at employment figures) whereas open source projects may have anonymous contributors with unspecified gender, I would expect there to be ways of estimating this. There are many companies that pay developers to contribute to open source projects, and many open source contributors who are known by name.
Demographic inclusion is important to measure for any project, to ensure it is getting the best developers from a large group (everyone) rather than from a smaller group (a specific gender). For an open source project where there's generally no salary to partially counteract disincentives for contributing, it is even more important to be able to measure any exclusion, whether real or perceived.
How does the community feel about demographic questions in general?