When AT&T launched the Motorola Backflip a few months back, it was largely a disappointment for those hoping for a solid Android experience on the network. The Backflip had a number of faults, among them was that the carrier had locked users out of installing apps manually. That is, apps that are not in the Android Market. Now the carriers second official Android phone, the HTC Aria, is also
unable to install non-Market apps
Android is an open mobile operating system, which allows manufacturers and carriers to modify it as they wish. It also allows users unprecedented control over the phone's software. In the case of AT&T however, it seems apparent the users are not to be trusted with that kind of power. Being unable to install external applications locks users out of many beta and pre-release apps. For example, the hotly anticipated Audible app is available only as a standalone beta that must be manually installed. The Swype keyboard replacement is in a similar boat. The Dropbox app was also released in this way before it hit the Market.
As a general rule, we're not in favor of taking options away from users. It feels like AT&T isn't understanding what Android is about. This looks to be a pattern for AT&T now, and we can expect this sort of behavior from them in the future. It is technically possible to use the Android SDK to install third-party software, but the process is complicated. Average users are going to be left out in the cold.