Valve has taken the lid off of what everyone already expected them to eventually announce, the official Steam client for iOS and Android.  While still in beta (you must install the app and sign in to ask for an invite into the beta test) it looks to be very polished.  From Valve:

Chat and participate in the Steam community, catch unbeatable Steam sales.

With the free Steam app for Android, 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.

Be sure to grab this now, login, and get yourself invited.  I personally hope to see this also on Windows Phone 7 at some point soon, as this could give Microsoft a good run when it comes to the mobile gaming experience and their Xbox Live mobile services.

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