I don't know any way to save Google Maps for offline use on my un-jailbroken iPhone.
Search for the app 'OffMaps'. It isn't great, but it is the best one I've found so far for off-line street maps.
If you have a real iPhone, I suspect that at least some of the navigation apps like TomTom, Navigon and Magellan work without Internet connection, but look before you leap.
-john