There is an app called AndNav2. It has the ability to pull the maps from the net and save them for offline use.
There were actually two apps. The original AndNav operated more like Google Maps, and didn't provide car-like navigation, with TTS and turn-by-turn prompts. AndNav2 incorporates this functionality and allows the Android device to act more or less as a car nav unit.
I think they just recently extended support for the US also. I used it today and have had decent results in Florida.
http://www.andnav.org/