I have an old SIM that I have used in an unlocked iPhone 2g. Everything works well but I cannot update the OS without risking bricking it. I would like to buy a new 3GS (Ebay seller says it is unopened = locked).

Will I be able to put in my old SIM card into the 3GS iPhone without activating it through ATT? I have a sweet phone deal from years ago and don't want to lose it.