I think it's more for the indie developers. 50% of the 130 games on my Steam account are indie or old DOSBox games (literally - an old game running in a packaged version of DOSBox). A lot of those indie games already have Linux / Mac binaries (World of Goo, Gish, etc.) if you buy them through other channels and many more of them can work using DOSBox and similar techniques. Some of the big franchises are able to work under WINE or have native binaries written for them.
Just because a Steam client is being written for other OS does not imply that they've successfully ported the entire Goldsrc / Source engine over to Linux, or ARM, or anything else. If you have a huge online store, the biggest digital software distribution client on the planet, and a vast proportion of your software can run on another OS in some fashion (and thus generate you more money from making people buy the other OS versions and/or encouraging takeup of Steam even if only for a single game), then making a Linux client makes a lot of sense - especially as the client is undergoing a massive rewrite already and it's where they want you to buy their games from in the first place, and making new software cross-platform to x86 OS's is pretty simple if you're using WebKit and other cross-platform libraries like the new Steam client does.
I think we'll see either "dual-OS" games (i.e. buy the Windows version and get the Linux / Mac version free / for a small fee) for the most part and maybe some "Non-Windows-only" games if they can talk existing indie developers into selling their existing software on Steam. I wouldn't hold my breath for ARM at all, or even any ports of games that don't already run in Linux. It's just a way to increase their capture of an existing, niche, games market by offering things that already exist in a well-known, convenient place that might make you spend a few dollars more than you normally would.