To counter anecdotes with more anecdotes, I went to a code camp a few weeks ago, and there was an overwhelming number of laptops running Microsoft OS's. When I pulled out my mac book pro one guy actually commented on it. I saw one other person with a Mac -- it was dual-booting Windows XP.
Of course, I couldn't tell you how many of the participants are open source contributors.
Among developers as whole I agree that Windows as huge percentage of the market share and that that probably won't change. But among open source developers I think it's different. For various historical reasons most open source development targets Unix and Unix like systems as a primary platform and OS X is currently the most popular unix like platform.
Of course, I couldn't tell you how many of the participants are open source contributors.