[Historiopode]

I would argue that your hopes would be better served by Unity Engine[1] and Mono[2] than by Steam. While high-budget releases have almost completely ignored the Mac and Linux platforms despite Valve's effort, the popularity of those tools has brought a very large number of mid-tier and indipendent developers to publish on them. While they are not as popular, they still help to make them look a little bit less like a desolated wasteland of work and productivity. :)

Let's also not forget that those same high-budget productions rely heavily on the current set of graphics (and audio) drivers, and there is hardly any effort to bring the Mac and linux up to par on that front.

[1]http://unity3d.com/

[2]http://www.mono-project.com/Main_Page