...why do male cycle commuters outnumber females? I haven't been able to find any usable statistics but from my observation of cycle commuters men appear to out-number women by around 10:1 

This is in a relatively bike friendly city, at least by North American standards (Vancouver Canada).  In my office, a downtown mixed office building, there are no women commuters.

There are many more women cycling in road clubs at the weekend and lots of women committed to cycling groups and promoting cycling.

Is there any reason why women don't commute by bike and is there anything that can be done to boost the numbers?

(rephrased to make this a more useful question - rather than a friday afternoon thing)
Oddly the well publicised bike counts down here don't seem to be quite so free with their results.

I did a bit of counting back in 2001 and got about 20% female cyclists. Fewer on major roads, more on bike paths. 1300 cyclists over 3 months in a perimeter count (because the official stats only counted 5 major entry points). Interestingly despite the compulsory helmet law only 74% wore helmets (I didn't count "helmet on handlebars" as wearing a helmet).

In Melbourne there are stats here, the "cycling to work" and "cycling in Melbourne" pdfs. First graph shows approx 25% female, and in the outer suburbs mostly under 20 years old, with a broader peak in the inner suburbs (interestingly, fewer school age kids than the suburbs)

and a second chart showing that as cycling percentage goes up so does the proportion of female cyclists - women are more likely to ride if a lot of men do.