As I understand it, the number is borne out in several studies.
The number comes from the 2002, 2003 and 2004 FLOSS surveys, which
took place in Maastricht [1]_, the US [2]_ and Asia [3]_,
respectively. The numbers are:
2002 Maastricht: 1.1% women (~2800 participants)
2003 US: 1.6% women (1493 participants)
2004 Asia: 1.5% women (138 participants)
.. [1] http://www.infonomics.nl/FLOSS/report/Final4.htm#_Toc13908243
.. [2] http://www.stanford.edu/group/floss-us/stats/q38.html
.. [2] http://oss.mri.co.jp/floss-asia/summary_en.html