To expand on exploring this, the blog Sociological Images, posted the following chart:
What strikes me as interesting about this chart is that some of the traditionally male-dominated fields, like construction, agriculture, mining, are some of the closest to equal in terms of pay, with construction at over 90 cents to every male dollar.
While fields that tend to have more women, like education and health services, retail, and administration, fair pretty poorly, between 75 and 80 cents for women to one dollar for men.
I'm not sure as to the cause, but I could hypothesize that fields that have attracted women for a number of years, now, feel no need to pay them equal wages - they already have women who will gladly accept their lesser earnings. In contrast, in order to attract female employees, some male-dominated fields may have increased earnings.