Steam now has a lot of casual games on it's service, i.e. Zuma. So this isn't just hardcore 3D gamers we're looking at.
It pretty much must be -- I can't see a significant number of people signing up for Steam specifically to buy casual games. Most folk sign up because they're hardcore gamers and the latest shiny nonsense that they want forces them to use the service, surely?
Now it might be that Steam users have an amount of interest in the casual fare that the service also serves up (in fact its very presence on there strongly infers that they do) -- great, but that tells us next to nothing about the wider casual market and what hardware the average gamer has.