AT&T seems to be doing something about the multitude of iPhone jailbreaks and iPhone unlocks which allow users to use iPhone on another network. The most recent thing is sending an SMS to users to upgrade their devices to a latest update available from Apple.
However, don’t fall for the upgrade message, it is a trap. The iPhone Dev team just tweeted that the message sent by AT&T (see above) is a trap and users should not upgrade their devices, specially those who have unlocked their iPhone.
The new update will possibly break the unlock and jailbreak, rendering your device unlockable till an update is available. So please stay away from it if you want to preserve your iPhone unlocks.