On my Mac (the horror! the horror!) I can log on, purchase and download the games that are released for Mac. I can even play them.
The trick is that once the Steam client has been ported, each individual game developer chooses whether to invest money in porting their awesome creation to OSX.
If Valve ported Steam to Linux, that would open a similar calculation for the developer. It would also mean that indie developers could develop on the Linux stack and sell their games to those who run Linux. Given careful selection of libraries, it's possible to run the same code on Linux, OSX and Windows. It would be sweet. But it depends on whether Valve thinks there would be enough money in the Linux market to pay for the development of a Linux client.