Why only Nvidia? Intel improved their open source drivers thanks to Valve, and AMD pushed some updates related to Steam as well.
And, while Steam officially support only Ubuntu (at least for now), Valve doesn't do anything for stopping it to run on other distro. They even modified the license to explicitly allow repacking, so that others could put the client in their repo.
They host on their site a .tar.gz with the Steam installer for distro that don't support .deb too.