[JoshTriplett]

SIM cards make it easy to change phones, by moving the SIM card to a new phone. CDMA phones make this hard, and sometimes impossible. They also make it a little easier to change carriers, since you can just switch the SIM card. It'd be even easier to switch if phones had that functionality built-in, so you could sign up for a new carrier and switch entirely via the phone, but in that case I think you'd find that carriers frequently broke that functionality.