If you assume a company doesn't want to port games, but decide to support linux at the beginning of their development cycle, we'd really need to wait a few years for proper titles. That is making the assumption they are interested right now though, which is less than likely.

However, there are currently many indie games available for linux. Even some well-known hits, like Kerbel Space Program and Surgeon Simulator (by chance, they both happen to use the Unity engine, which natively includes linux support).

Just out of interest, what was the timeline for OSX steam like? I don't use a Mac, so I'm not sure.