This is interesting -- according to a Steam user who got to Valve headquarters in Washington, the popular game network may be looking at extending itself out onto the iOS platform. That doesn't necessarily mean that there would be a straight-up Steam client for the iPhone and the iPad (although how awesome would that be?), but rather, it sounds like Steam is looking at integrating either its social networking or achievement integration features out towards Apple's platform, in the form of an app or something else.
Whatever Valve is working on, we can't wait to see it. Steam coming to Mac OS was a huge boon for Mac gaming, and the success of iOS was, I believe, a major reason that happened in the first place. Whatever Valve wants to do on iOS, there's no doubt it'll big a big bonus for customers.