Despite Gabe Newell's harshly critical comments of Windows 8 as a closed platform, it appears that the platform's usage has already outpaced Mac OS X.
According to Steam's Hardware & Software Survey for November 2012, Windows 8 users account for 4.69% of overall Steam users, making it the fourth most used OS. In comparison, Mac OS X users sit a little above 3%.
In contrast, Windows 7 experienced a 0.84% decline, presumably from some users upgrading to Windows 8.
Unfortunately, since the Hardware & Software survey is opt-in, this may not be an accurate reflection of the overall breakdown OS breakdowns of Steam users. The survey also only shows off percentages, so there aren't any actual figures.