You can turn cellular data off on the iPhone and still use the GPS. Airplane mode, however, will shut off the GPS. What really kills you is that visual voicemai

You can turn cellular data off on the iPhone and still use the GPS. Airplane mode, however, will shut off the GPS. What really kills you is that visual voicemail runs over the data network, and it's non-trivial to turn that off. I spent last month backpacking through Europe with my iPhone, using GPS every day, without unexpected charges by leaving data off. A friend of mine on the other hand turned data on for 5 minutes to check his email for a phone number. That triggered 10mb voicemail download and a nearly $200 bill for the month.