38 percent of married American women now outearn their husbands. This is up from less than 25 percent in the late '80s. Keep in mind that that number includes couples in which the husband isn't working.
Cohabiting couples were not included in the study/survey cited, though. Same-sex couples were also not included. Still, it is interesting, if you can wade through the wonk-talk - it is 53eight, after all.