These days OSX seems to be pretty hot for game dev, certainly if you want to do any iOS stuff, and OSX is creeping up as a platform too, with Steam now available there. No doubt windows will be king for a while yet though.
[1] http://en.wikipedia.org/wiki/Making_of_Doom#Building_the_gam...
And more importantly, as it relates to DHH's statement, excluding Windows developers isn't like some odd niche. It's probably a healthy percentage of the best developers in the world. Fortunately for DHH he's at a webdev shop where pure coding skill is probably less important than cultural fit.
I would argue that, at most programming jobs coding skill is less important than cultural fit, once you've passed the level of "reasonably competent developer."