OS X also has actually usable video drivers, where as on Linux you pretty still need the binary NVIDIA drivers for any non-trivial graphics stuff.
Common, OS X also uses the "binary nvidia drivers", Apple doesn't create it's own version of Nvidia drivers, are you trying to inject some dichotomy between open source as a movement and the quality of the binary drivers on linux and os x.
Man do I hope they give Linux some love too. Counter Strike Source is the only reason I still maintain a Windows partition. Running it on Wine just hasn't been reliable enough. In my opinion no game company should ever worry about supporting all distros. They should just say were only going to officially support Ubuntu or OpenSuse or Whatever and be done with it. Most likely it will just work on most modern distros anyway and even if it didn't, switching distros would be a small price to pay. Certainly better than having to run Windows.
This begs two questions. Will the OpenGL renderer be available for Windows? And if not, is there some kind of Wine equivalent for running OS X programs? I can't think of one off the top of my head, which I find surprising, given that it's a lot more like Linux than Windows is. Surely if FreeBSD can run Linux programs...? Or am I being naive?
I highly doubt that we will see Steam on Linux anytime soon. Linux just doesn't have the consistency that OSX has in terms of 3D support. Performance too is far better on OSX than Linux shown in a fairly recent Phoronix test.