The website requires you to log in so there isn't much to explore without creating an account. Either way, AT&T Apps Beta is a platform for connecting highly involved AT&T customers with access to the latest and greatest mobile applications.
Can you see a pattern here? Another large organization using Drupal to build an add-on site. Once an organization has its feet wet with Drupal after implementing a couple of micro- or add-on sites, you start to see Drupal bubble to the top of the organization or to larger and larger web properties. Either way, it is great to see one of the largest technology companies use Drupal. Another win!