Not sure about the mobile maps API, but in the professional version of the gMaps API, you can download all the map tiles and not have the need for a network connection to browse the maps.
If you just DL the tiles for the areas of interest, I'm sure it could be done on a phone. DL'ing the whole US might cause a storage issue for phones though (though I hesitate to say this since many phones now have rather large storage capacity and SD cards, etc.)
EDIT:
From the article:
Google Maps Navigation was built from the ground up to take advantage of your phone's Internet connection.
If you just DL the tiles for the areas of interest, I'm sure it could be done on a phone. DL'ing the whole US might cause a storage issue for phones though (though I hesitate to say this since many phones now have rather large storage capacity and SD cards, etc.)
EDIT: From the article:
Google Maps Navigation was built from the ground up to take advantage of your phone's Internet connection.