Steam needs a statistic for Wine/crossover. I play tons of games on my mac via steam through crossover. It's crazy that multiple layers of translation work so well.
Mac market share is closer to 20% when you exclude machines installed in offices and enterprises. Some estimates put the number as high as 30% in North America.
The fact that the Steam number is as high as it is, despite the extreme lack of compatible content, is noteworthy.