I'm curious to know if there are similar biases in the workplace - if anyone's dared to document it.
There is data in the Wikipedia article Personal Income in The United States which I believe addresses your question. The results are not a surprise to me.
As I reviewed this, I see the data is a bit old, 2006 for this chart. I don't think the numbers have changed much, though.
There is a quite an extensive literature on the racial/ethnic and gender differences in earnings in the US and elsewhere. There is comparatively less of this work done on earnings differences in sexual orientation, but starting with a great paper from 2001 that relied on a novel use of census data from the 2000 US Census, it was shown that gay men earn roughly 16% less then straight men. Unfortunately you need access to JStor to read it online but here's the link:
For more on gender, race, ethnicity, and sexual orientation, search the labor econ literature on http://econpapers.repec.org or http://scholar.google.com. You can also hit the BLS site for tables of wages and other labor market gaps by demographics from the Current Population Survey at http://bls.gov/cps/earnings.htm#demographics