In 2006, ~30% of BSE degrees conferred were to women
In 2005, the number was ~36%
http://registrar1.princeton.edu/data/dgconf.pdf
One little word of caution interpreting the above data, though: these are the degrees given to the classes that
graduated in 2005, 2006, etc. Since then, there've been a number of changes, including a greater emphasis on engineering (especially getting women into the field) and a new dean of admissions. The numbers for '08, '09, and '10 may thus be pretty different.
Also, like ec1234 notes, more relevant might be the particular field of engineering you're looking at. While majors like ORFE might be close to 50/50, CS isn't known as a hotbed of women. This generally seems reflective of a national trend in computer science. The NYT actually had an interesting piece on this:
http://www.nytimes.com/2007/04/17/sc...e5e1&ei=5 070