Valve regularly runs surveys of steam users, where they profile many different things about the hardware. They're nice enough to publish the results:
http://www.steampowered.com/status/survey.html
Some of these provide a way to detect Wine users. Everyone using the "Wine waveout" audio driver, for instance, must be a Wine user. Unfortunately, valve lumps all of these into "other" on the survey results page.
Worries about sampling bias aside (maybe Wine users are less likely to do the steam survey), if we had the raw data from Valve we could determine the actual percentage of Steam users playing through Wine.
Looking at the data we do have, we may be able to make a good guess from examining the video card driver name field. Excluding NVidia, ATI, and Intel leaves just over 4% of respondents using an "other" video driver like the Wine one. If even only a quarter of these "others" are using Wine, that still gives us 1% of the entire 10 million+ Steam user base, translating into hundreds of thousands of users.
Does anyone know whom we can contact at Valve for more specific results?
Thanks, Scott Ritchie