That's a question for Apple.
But, they would not be able to put a SIM in it, unless it was unlocked. On locked phones, only the SIM that came with it would be the one that would work.
If your phone was stolen, and you know who has it, I would report them to the police, as well as Apple and AT&T.
To get a new SIM to work, it has to be programmed with your IEMI number. Both Apple and AT&T can do that, but someone just off the street trying to put their sim card in is not going to get it to work. At least not without a whole lot of work.
unless you had the IMEI on your computer, called AT&T, and BlackListed it, so that anytime someone connected it with their network (if you bought your phone pre-1.1.2, it might be worth a shot calling T-Mobile also) they would be tracked down and found and (probably) arrested.
i just bought a used iPhone from a guy i know he had a prepaid sim card in it. i put my regular sim card in it and it works fine.i don't think its unlocked. my question is i can use wifi and use the internet fine even though it sais it cant activate edge, if i go to iTunes and say i don't want a data package and i don't use text messaging will there be some type of charge on my bill for data for just useing the wi fi. or does someone know a way i can use iTunes without registering it , the phone has 1.0 firmware which i have never even seen anyone talk about. thanks if you can help