As the go-to guy for most tech related questions in my office and my personal circles, I should know this, but, when you use a GPS app such as Google Maps with the iPhone 4, do you pay for data?

When you use Google Maps on an iPhone, it will download map data and whether you pay for that data per MB or monthly depends on your data plan.

You do not pay for data received from satellites, i.e. GPS data. I don't know about the iPhone, but my Android can even still use GPS in airplane mode, when the 3G/LTE radio and the WiFi are disabled. The GPS receiver is a passive part that only receives and is unrelated to the regular wireless network, except that having access to the wireless network speeds up the initial satellite lock because the phone can download satellite position data off the internet.

I think the Android version is different than the iPhone version, in that I believe Google Maps can work offline on the Android.

Co-Pilot, an app for Android, downloads the maps first and saves them on your smartphone. That way, you don't need to be near a cell tower for the GPS to work. It's a paid app that has been worth it for me. YMMV.

The basic answer is yes, in that the phone has to have a data connection to keep the map updated. There may be an app that will allow you to download maps for a given area to the phone, in which case you would not necessarily have to have the data connection. I know this exists for the Android platform. I would have to mess with my wife's iPhone to see if it can be done....

Aaron