There is an absolutely fascinating article in tomorrow's Washington Post, "Salary, Gender, and the Social Cost of Haggling." Linda Babcock, an economist at Carnegie Mellon University, became interested in how men and women negotiate differently when female graduate students came to her and pointed out that the male students were teaching their own courses while the female students were acting as teaching assistants. Upon investigation she learned that the male students had each approached the dean and requested to teach their own course. JSM used the book Babcock wrote based on her research, "Women Don't Ask," in his negotiation course back when he was a professor at a b-school, and I highly recommend it. Her research goes a long way toward explaining the residual salary differences between women and men once education, experience, and job tenure are taken into account.
But the really interesting part of the article is the follow-up research, addressing the question of why women don't negotiate. It turns out that "across all the studies...men were always less willing to work with a woman who had attempted to negotiate than with a woman who did not. They always preferred to work with a woman who stayed mum. But it made no difference to the men whether a guy had chosen to negotiate or not." Some studies found that when women thought a woman would be making the hiring decisions they were much more likely to negotiate.
In other words, women don't negotiate because they don't get the same results as men do. They may even harm their career by attempting to negotiate their salary.
Women may be playing the game optimally, but that doesn't mean the game is fair.