Updates to fix the Stagefright exploit are beginning to roll out from Sprint, and now AT&T is joining with updates for a bunch of Samsung phones. The update, which includes a 'device security enhancement' is now pushing out from the carrier. The update is currently showing as available for the following devices:

If you want to check now for the update, head into your phones Settings, then System Update, and finally check for the update. Be sure to let us know in the comments below if you have already received the update.

Source: AT&T