Steam is a very interesting cloud-based service that allows users to play games on any PC and that maintains the state of not only your gaming, but your community as well. This model encourages more game play and a higher likelihood that users purchase additional services as time goes on. It also puts some control back in the hands of the users, who aren't burdened by DRM.
Some interesting statistics:
70 percent of users are running Windows XP
65 percent of users have Nvidia graphics cards
64 percent of users run Intel processors (versus AMD's 36 percent)
While these results are not surprising, I would expect to start seeing more Vista adoption simply to get the DirectX10 graphic performance.
Dig in to the hardware survey for more details