I just did this with the miniSIM card from my iPhone 4 (ATT) to my iPhone 3GS. First time it was not sucessful, so I pulled it out and repositioned the miniSIM card and it worked perfectly! (Note: In regards to the first post, this method does DESTROY the full size SIM card, but if you are taking these steps this card is MOST LIKELY deactivated and thus garbage. In no way *should* this harm the miniSIM card.)