I use the TomTom app on my iPhone and it works great when the cell network is unavailable. Unlike the iPhone maps and some other GPS mapping apps that rely on the ability to download maps real-time, TomTom has the maps stored in the phone memory. Admittedly, it isn't going to do topos, etc, but it'll get you around as long as the satellites are still there!


On a side note -- One thing I like about the iPhone "maps" app is that if you zoom in very close on the "map view", it will show you the county property lines and is usually accurate within several feet. Great for knowing what piece of land you're on!