According to AT&T Forum, a small update has been pushed out to their version of the Galax S II, please make sure you check your phone for any updates.

Statement from AT&T's forum

As some of you have already noted, an update is available for the Samsung Galaxy SII.

The update includes fixes for devices that may become unresponsive and/or power down while sitting idle on rare occasions. It also enhances security to protect against malware vulnerability. You will receive a notification on your phone to update.

NOTE - You must have updated to Android version 4.0 to receive notification of the new software update

In case you are wondering what's in the update. The update includes software fixes an issue where devices would become unresponsive or power down while idle. It also includes the requisite "security enhancements."

This is rolling out over the air, so hit your Settings app to find it or just wait for the notification to show up.

AT&T Forum