While number portability may have freed your cell digits, your phone is still a ball and chain, locked into one carrier's service. These subsidy locks keep you from walking away before the provider can recover that big discount you got when you bought the phone.Link
But it doesn't have to be so. If you have a GSM phone, you can unlock it and switch to any GSM network carrier (the big three are AT&T, Cingular and T-Mobile). You can also take an unlocked phone overseas (most of the world uses GSM) and use it on a local network to avoid paying for international roaming, or even buy a European phone (they tend to be ahead of us in cell tech) and use it here. Have an old phone lying around? Unlock it and keep it as a spare.