New images have surfaced today, which seem to confirm Valve's intentions to bring Steam and possibly several of Valve's games to the Mac platform.
If you missed the previous rumors, a user in the Steam forums discovered Mac style skin files in the new Steam beta. If true, this development could turn out to be a major new step in computer gaming for a variety of reasons. While many Windows gamers love to poke fun at Mac users, several developments over the past few years have left PC gamers questioning Microsoft's real intentions when it comes to PC gaming – see Alan Wake, etc. In addition, because OS X is based on Unix, we would expect that Steam would also be making its way to Linux as well.
Ultimately, this should be interpreted as good news for computer gaming in general simply because it expands the user base and also future-proofs gaming on the computer platform. Operating system market share certainly isn't set it stone and if Microsoft were to make a highly undesirable move, for example, it gives gamers alternative options. Of course, just because Steam looks like it's headed to Mac doesn't mean all the games in the Steam store are too. However, it might influence more developers to program for multiple operating systems.