Future Valve games will hit PC and Mac simultaneously, beginning with Portal 2.
Both PC and Mac versions will be bundled together, meaning that buyers get both editions of the game for the price of one--a feature that Valve calls Steam Play. Furthermore, games with support for Steam Cloud server-side save storage can be started on one platform (say, PC) and continued on the other (in this example, Mac).
"We looked at a variety of methods to get our games onto the Mac and in the end decided to go with native versions rather than emulation," said Valve's John Cook, explaining:
The inclusion of WebKit into Steam, and of OpenGL into Source gives us a lot of flexibility in how we move these technologies forward. 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.
"Checking in code produces a PC build and Mac build at the same time, automatically, so the two platforms are perfectly in lock-step," added Portal 2 lead Josh Weier. "We're always playing a native version on the Mac right alongside the PC. This makes it very easy for us and for anyone using Source to do game development for the Mac."
Update: Valve has provided us with a few more details on Steam Play:
[Steam Play] will allow Mac gamers to play against PC gamers and -- for our games and any of our partners who are interested in using the feature -- be able to obtain any game they purchase on either system without the need to purchase two copies. So, for example, if you already own Left 4 Dead 2 on the PC you won't need to purchase it again for the Mac.
Update 2: Unsurprisingly, Valve's official blog offers word that the original Left 4 Dead will be joining Left 4 Dead 2 and everything else that the company is bringing to Mac.