Problem with Google Maps is that there never is any reception when you need it. Although you are right - eventually there might be (10 years? 20 years?).
Except you could build pre-loading of map data into the app. On Symbian phones with Ovi Maps the app will download map data on-the-fly, or you can choose to pre-load it (for example the full maps for Europe or whatnot). I see no reason Google couldn't do something similar.
Google Navigation already does this, apparently. I was checking out one of the state parks in the Santa Cruz mountains a month or so ago. Naturally, I lost my cell reception once I got into the foothills. But Navigation continued to direct me, turn by turn with map, until I arrived at my destination.
I lucked out on GPS though - in some cases, the GPS signal has died just when I need it most. Darn Nexus Ones.
