I think XBLA has shown that great games can have long legs on that service. Things like being top rated, top download, friend lists and deal of the week can help take a successful game and keep it in people's minds for when they are looking for something new.
I feel that PC users are more hardcore, Meat Boy will have sold big in the short term because it was hyped and because of Steam sales, but I think that the numbers might balance out over the course of this year, when PC users are chasing the next big thing. I could be wrong, hopefully there are still casual Steam users who haven't gotten around to buying it yet.
I only paid 800 points for Super Meat Boy in launch window, but I would gladly pay 1200 now that I know just how much content there is in that game. It's quite amazing, and anyone reading this who hasn't played it yet should definitely try a demo if you can.