According to a forum post on AT&T's support site and some scattered user reports, the carrier's Galaxy S II variant is receiving a small update today. Rolling out over the air as you read these words, the software fixes an issue where devices would become unresponsive or power down while idle.
The update also includes the requisite "security enhancements." If you haven't updated your phone to 4.0 yet, well for starters, why not? Secondly, you'll need to do so first before you can apply the update. This is rolling out over the air, so hit your Settings app to find it or just wait for the notification to show up.