This is certainly great news. Let's not forget though, that for a company like Valve, development of their games cross-platform is only half the issue. Supporting them after release is just as much of a concern, and they may or may not hold off until they've got a solid grip on how to handle that sort of stuff.
That being said, with Mac already gaining official traction, it looks like there's definitely a strong possibility that this is going to happen. As their users and potential customers, we should keep asking and making our interest clear. Only when they see the potential benefit to Valve as a company are they going to go full steam ahead, so to speak.
Personally, I think it will not only be good for the operating systems as gaming platforms, but also for Valve - this can open up a game market on both the Mac and Linux platforms, and Valve would be the first, and for a long while the only, big player to operate in it. Remember, a few years ago nobody thought Steam would ever reach >25M active users - on the 'dead' PC gaming platform, no less.
Today, all the big publishers are kicking themselves for not doing it first.