This comes along with news that Portal 2 will be the first Valve product to release simultaneously on Mac and PC. Also, Valve's director of Steam development, John Cook, is quoted in the release with some details about how some of this will work.
If I may get "sensational" for moment, note that Valve considers the Mac to be "tier-1" but not the PlayStation 3. Burn?
"We are treating the Mac as a tier-1 platform so all of our future games will release simultaneously on Windows, Mac, and the Xbox 360. Updates for the Mac will be available simultaneously with the Windows updates. Furthermore, Mac and Windows players will be part of the same multiplayer universe, sharing servers, lobbies, and so forth. We fully support a heterogeneous mix of servers and clients. The first Mac Steam client will be the new generation currently in beta testing on Windows.”
Furthermore, Valve is rolling out something called Steam Play, which is a backend feature that will allow players to play games on Windows or Mac for one purchase price. Again, Portal 2 will support this, along with Steam Cloud, so your saved games will transfer between platforms.
So unified multiplayer servers, simultaneous releases, and the ability to play on Mac after buying a Windows version of the game. Yeah, that sounds about right. A little too right, even. I'm not sure how this will do, since it seems like the whole "Mac gaming" thing comes and goes every few years without making any waves, but this all sounds kind of awesome, provided it has enough developer support to offer the right variety of games.