Do women call in sick more often than men? This MSNBC article says they do.
The inference for women's wages is that their skills depreciate or they cost the company money either by hiring a temp or having to pay other workers overtime.
This link writes that women are less productive than men. This link is a learned 69-page paper that explores productivity in the US and around the world. I don't know what to think.
Finally, from Superfreakonomics, "Though the sample size is small, women who underwent surgery to change into men earn more after the gender transition while men who became women earned on an average 1/3 less than their previous wage." (pages 47-48. I love how statistics are shown to the readers.)