"SIM cards use the same tech as smart cards (the Visas and AMEX Blue cards with the the chip). It's also the same tech that DirectTV uses to identify recievers. You can buy the equipment to read/write these cards (you know how your phone came with a credit card sized card that the SIM punched out from, you'll probably have to tape it back in that cutout to fit it into the reader/writer). They are legal to own, but it is illegal to clone a SIM for use (FCC regulation). This also applys to changing the ESN numbers on TDMA/Analog/CDMA phones. "
Fwiw.