I am very happy to now be able to direct readers to the APA Committee on the Status of Women webpage, created by the determined Peggy DesAutels (with the helpful contributions of philosophers and assistants including me!), for data including my original report, "Women in the Profession," from 2006. There, I hazarded the estimation that roughly 21% of philosophy professors in the United States are women. You'll see the 2011 update posted there, too.
I have been asked how one might cite the original 2006 report and the 2011 update; now that the information is hosted by the APA CSW, you're welcome to refer to either as an online publication, cite the apaonlinecsw site for the URL, and cite me as the author (since I wrote the 2006 report before joining the committee). While you are on our google-site for the Committee on the Status of Women, feel free to look at the other content, some of which was recovered from the APA's old website, but much of which is new content for our offsite webpage.