Steam will continue to grow, because of several reasons. But the two major ones that I think they did this year was to increasingly tie retail releases with steam, meaning people will be creating accounts just to play the games they bought in the store and their release on mac. Which is a platform that may be small, but given Apples fortunes most assuredly is seeing nice growth.
The other thing I've noticed with my friends and this is purely anecdotal, is that despite the fact that we all have had accounts since the release of half-life 2, we're now only coming back to the service to actually buy games.
I've bought more games on steam this year then all other years combined. Why? Because the pc retail sections keep on getting smaller and its hard to find a lot of the small games on the shelves. While I still by the big pc releases at retail stores all my indie and smaller releases are now bought on steam. Games such as the updated Monkey Islands, Machinariam, Aquaria etc.
