Unconscious bias toward recommendation letters written by men?
I couldn't find any recent (written after 2000, that is) study that directly addresses the issue of how the recommender's gender influence how the letter is perceived. However, I did find one interesting survey study (doi: http://dx.doi.org/10.1111/j.1468-2389.2009.00453.x) that asked experts in personnel-related professions how they think about LORs. One item asked them to rate on a scale from 1 (strongly disagree) to 5 (strongly agree) the following statement:


  I believe that applicant, referent, or reader gender may influence how one interprets the contents of LORs


The average response was 2.86 with an SD of 1.22. About 42% strongly agreed, and 43% strongly disagreed. Clearly there is some level of controversy when it comes to the role gender plays in LORs. The study did not, however, ask any question directly about the referent's gender, which makes it even harder to draw any specific conclusion for our purpose here.