That five percent figure comes via the latest Steam survey, which analyzed the digital distribution platform in July 2010. It's down from 8 percent in June, but that spike was likely thanks in large part to free Portal. Interestingly, almost 47 percent of the folks on Macs who accessed Steam during July did so using a MacBook Pro.
Also good to keep in mind: back in July, only a small fraction of the games on Steam were available for Mac users. Since then, an ever-growing catalogue of Mac-compatible games has recruited even more Mac users into the army of bloodshot-eyed fun seekers.