I was reading something I saved earlier for possible blogging, about the scarcity of women at the top of corporate echelons, and came across this:
She describes a corporate environment that offers much more latitude to men and where the bar is much higher for women. In her view, men tend to be promoted based on their promise, whereas women need to prove themselves multiple times.
I doubt there's any research on this question, but what she describes there might fit some cases I know about. That would explain why promotions come more slowly to women than to men. The "promising young man" vs. "the woman who has earned her spurs."
As I mentioned, this is pure speculation on my part. What's interesting about the idea is that if it turns out to be true it probably wouldn't be based on overt discrimination or anything of the sort but on something more vague.