There's an unofficial leaked list of games that *might* end up being on Steam for Linux, when it comes out.
The list seems a bit underwhelming at the moment, and devoid of any big modern names. Some of them, like Defcon and Uplink, definitely already have Linux versions and I'm pretty sure in the case of those two, it's a binary replacement you can already apply over the top of a Windows version.
But it's almost certainly not complete, as Valve have been using L4D (or was it L4D2?) as the basis of their Linux push for a while now. According to their official blog there'll soon be a private Steam Linux beta supporting one Valve game (probably L4D2 then... but it would be nicer to see TF2 on Linux, in my opinion).
I'm fascinated to see how this will work. They've restricted it to Ubuntu 12.04 and later, which cuts back at the insane amount of heterogeneity in the Linux ecosystem, but I think they'll have to expand it beyond Ubuntu later. There is also the huge problem of substandard graphics drivers on Linux; nVidia's are rubbish and frequently make KDE/Qt4 feel very sluggish, but nVidia's the only show in town if you want to run 3D applications on Linux, so dire is the quality of ATi's drivers. It will be interesting to see if Valve are important enough to change hardware vendors' priorities. Let's hope so.