UPDATE: I just realized I completely forgot to mention that the only people who can access this right now are part of a closed beta. From the press release:
Those who wish to join the beta just need to download the Steam app (free of charge) and log in to their Steam account via the mobile app to express interest in the beta. Gamers will be added to the Closed Beta as the service ramps up.
So, hey, do that.
Valve today announced the Steam mobile app for iOS and Android platforms, a free-to-download program designed to let the Steam power user check their friends list, chat with said friends, and yes, buy new games pretty much any time it tickles their fancy.
This means that never again shall you be forced to miss out on one of Steam's many, many, many sales because you happened to be walking around out in the larger world, and not handcuffed to your computer for the duration. In effect, downloading this app ensures that you will never, ever be able to escape the opportunity to buy games on Steam.
You didn't really like having money anyway, right?