Valve are branching out to mobile phones and have a Steam app for iOS and Android. You will be able to connect with friends and see what they are up to and even chat to them. You will also be able to shop and browse the Steam store. There is no word if any games will be available through this at the moment, but being Steam I'm guessing it's just a matter of time. There are plenty of little indie games on Steam that would be suitable as phone games. The app is currently in closed beta form. But Valve will be sending out more invites as the it's popularity grows.
Gabe Newell has commented on the app saying:
“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.”
By that he really means is "We can now rape you wallets on the go. Where ever you are, you can not hide. We will find you and we will take you money. Here at Valve have come up with the most genius evil scheme which is to bankrupt the world little by little and slowly but surely we are doing it."