Valve has released Steam Mobile for iOS, a free app for the iPhone. According to Valve, "With the free Steam app for iOS, you can participate in the Steam community wherever you go. Chat with your Steam friends, browse community groups and user profiles, read the latest gaming news and stay up to date on unbeatable Steam sales."
That's the promise of the app, anyway; Steam Mobile has launched in "limited beta," meaning you first need to input an activation key on your PC or Mac. Entering your username and password into the Steam Mobile app is apparently enough to "express interest" in the beta, but for now that's pretty much all the app does until you receive an activation key.
It's interesting that this app was approved; Apple has traditionally frowned on apps that require users to register for access in this manner, so it's odd to see Valve granted an exception.
Also worth noting is that Steam Mobile will not serve as a front end to Steam selling games for the iPhone. All Steam Mobile will allow you to do is access the community and buy games for your Mac or PC from your iPhone.