The Dearth of Female Bosses

I found this article in the NEW YORK TIMES about the dearth of women as bosses in the US. We seem to have this problem in Australia too. Why do you think this is? I would love to hear your comments on this? What is it going to take for women to rise in business? Sure, there are some women who have navigated the glass ceiling, but they are few and far between...what is holding women back?