No. According to the Steam hardware survey, for November 2012 Win8x64 has a 4.25% install base (less than WinVistax64). Win8x32 has .44%, so Win8 has a combined percentage of 4.69%. By comparison, the top four OSX versions have a combined percentage of 3.26%. WinXP has a combined percentage of 10.83%, and Linux isn't even a blip on their radar.
Source:http://store.steampowered.com/hwsurvey