Several sites are reporting that third party Steam apps have been disappearing from the Android Marketplace following the release of Valve's official application last week. Because Steam's API is freely available to any developers wishing to make their own applications, this is a move no one saw coming—if Valve are the ones behind it. There's no proof of that, though it's clearly more than just a coincidence.
Valve is expected to continue adding features to the long-rumored Steam Mobile app for iOS and Android. Although now that every other Steam app has been removed from the market (besides Staticon, which we're told lacks many key features), we no longer have a choice of which one to download.
Hopefully Valve will release an official statement about this. They may have had a very good reason (yes, Valve gets the benefit of the doubt). You might disagree—let us know in the comments or on Twitter.