Wiley Author Survey on Open Access Access 2012:
http://www.eurekalert.org/pub_releases/2012-10/w-waw102612.php
http://www.slideshare.net/WileyScienceNewsroom/wiley-14895586
Out of the 10,673 respondents, 32% had already published in an open access journal, defined as "free for all to read, download and share online and the author, their institution or funding body pays a fee to ensure that the article is made open access. The highest proportion of open access authors came from a medical background (28%), closely followed by biological sciences (24%)". 

I find these discipline-related differences intriguing...