Valve have just announced that a Steam app is incoming for iOS and Android. It'll support chat, groups and screenshots. You'll even be able to purchase games when on the move.
Lord of Valve, Gabe Newell seems keen: "Seeing which of your friends are online and playing a game, sending quick messages, looking at screenshots for an upcoming game, or catching a sale - these are all features customers have requested. Mobile is changing way people interact, play games and consume media, and the Steam app is part of our commitment to meet customer demands and expand the service functionality of Steam to make it richer and more accessible for everyone."
I can imagine the drunken 3G purchases/messages already. Apologies in advance friends list.