One of the things that sets the Google Android platform apart from Apple’s iOS is the fact that you can install third party application that aren’t available in the official marketplace. This makes it easy for developers to distribute apps any way they see fit, or to offer people an opportunity to test beta applications without making those apps available to the general public in the Android Market.
The ability to install third party apps from APK files downloaded or copied to your phone isn’t turned on by default. You typically need to navigate to the Applications menu in your phone’ settings and check the box next to “unknown sources.” That’s easy enough on most devices — but not Android phones from AT&T, as it turns out.
AT&T has just launched its second Android handset. And for the second time, the company has removed the “unknown sources” option altogether.
The two affected phones at the moment are the Motorola Backflip and the HTC Aria. Both Motorola and HTC have made a number of Android handsets for other carriers that do support installing apps from outside the Android Market, so clearly AT&T is the common denominator here.
via Android Central