I'm thinking about buying a used iPhone 3G and not activating it with a wireless plan but simply using it as a GPS device (car navigation, geocaching, etc.). Will I be able to use its GPS capabilities without it being on a live mobile phone network? If so, anything special I need to do such as jailbreaking and/or unlocking? Is there a way to turn off the phone features while keeping on the GPS features to increase battery life? Any advice would be welcome. Thanks!