you can restore as long as youre not using the sim unlock. the best way to find out if your phone is going to be bricked during restore is to first check the IMEI number on the back of the phone. then go to settings/general/about and check the IMEI number there. if those numbers match then youre in the clear...there is only a 20% chance or less of your phone being bricked.
i had jailbroke my phone and updated the software and now i have a perfectly functioning 1.1.1 stock iphone. i had no problems during the restore.
again as long as you are still using your original ATT sim, and haven't unlocked the phone to use a sim with another network, you should be in the clear.
