n0nsensical

As far as I know, any given iPhone needs only be activated once. After you've done that you can use any valid AT&T SIM card in it. So there's nothing to worry about for a used phone. The store isn't going to tell you this because they only deal in new phones. From personal experience I can tell you Cingular/AT&T stores are generally worthless for anything that doesn't involve you paying a thousand dollars for a contract. The only thing unlocking/jailbreaking does for you is give you the ability to run unauthorized apps or to use the phone with an unauthorized phone network. If you already have AT&T service you can simply swap in your SIM card and it should work fine, of course I recommend you test it out first before handing over any cash for it.

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"Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded." --Abraham Lincoln