The iPhone GPS works fine out of range of either or both WiFi and Cellular coverage. You cannot put the phone in "airplane mode" however, since that does disable the GPS. This does mean that your battery life will be a bit shorter, as the phone constantly searches for Cellular service. Nonetheless, with one of the apps that allows you to download maps, you do not need to be connected to see your location on the downloaded map.
You can get an external GPS unit for your iPhone and turn airplane mode on. The Bad Elf works great for an application like this, and only draws a small amount of power from the phone to work. There is also a blue tooth model that has its own battery, but works the same way.
I use a Bad Elf for supplemental GPS capabilities in the cockpit with my iPad, and this was a nice secondary surprise for usage with my iPhone.
