Got a Flipside that is supposed to be unlocked. How can you tell if it is unlocked or not?
Put your non-ATT SIM card in it and see if you can make a call. If the phone isn't unlocked, you should get some sort of message about needing to have ATT service.
Also, I believe that if you do what FlipsideJewel mentioned above, and your phone is not unlocked, it will ask you for an unlock code.