Another side to this I see at my work is that it's the men and women without children who (generally - there are exceptions) stay back late and come in on the weekends when it's really busy and also who are willing to travel interstate and overseas. Hence they end up moving through the ranks quicker and get bigger bonuses. It's a tough one. As a manager I have had to put people forward for promotion and it's rarely been mothers - it's been those who volunteer to do extra work. And as they do extra work, travel, etc, they end up with more experience and the cycle continues.
I think the key to closing the pay gap lies with men taking on more of the child rearing responsibilities. If, of course, that's what a couple decide is right in their relationship.