Ever wonder what exactly that tiny SIM card in the side of your AT&T/GSM iPhone or iPad 3G is and what it does? The short answer is, it's a Subscriber Identity Module, and it is a small circuit board which is placed in your iPhone (or any GSM phone) in order to identify it to your carrier. It's why you can swap SIMs in GSM phones and your phone number, voice, and data plans are swapped right along with it. Unfortunately, it's also why you can't just take a SIM-locked AT&T iPhone and run in on another carrier. For the long answer, stay with us after the break!