Depends on what you call "developers". I believe that most developers in the gaming industry still uses Windows as their platform.
> are either writing Windows software
I assume most people in the game industry need their game to run on Windows or use hardware that only works with Windows (like a dev kit for a console).
Unless you are talking Web and mobile games, in which case, most of the web gave devs I know use Mac and so do most of the mobile devs (thanks the the difficulty deving for iOS on Windows).
But that is why I called it an "anecdote"... your milage may vary.