It is certainly frustrating when you are using a navigation app and it shuts off because there is no data connection. Copilot is changing that, they are offering offline maps for free, so your cellphone virtually becomes a standalone GPS. If you want voice navigation and 3D maps, it is a one time $20 charge and real time traffic updates will tack on $10 per year. But it's pennies if you figure what you save on buying another GPS for when you drive into the non cellphone wasteland when you cant even call anyone to get directions.
http://www.engadget.com/2012/02/26/copilot-gps-app-ios-android/