As others have said, simply turning off data roaming avoids any data charges and still allows you to use it as a phone and to use the net at wifi hotspots.
What surprised me is that even with data off, I was able to use the location tools in TripAdvisor and Lonely Planet (i.e. click on "what's nearby" for hotel/restaurant locations in your immediate vicinity) and they didn't cost a cent. I guess the GPS function is different to the data, but the iPhone Maps function needs you to be wifi connected.![]()
Mapping via cell tower is surprisingly accurate. We use cell tower databases for one of our products that customers use, and it's actually very good for a lot of places. This is often all that's required, Data is not necessary if the phone has a database of locations stored locally.