FYI - just got the Google maps 5.7 update for my EVO 3D. Go into the labs (menu.. more.. labs) and select the download map area. Now, on the maps page, you can download map area for offline viewing!
With the box being a 20mi x 20mi area (400sq/mi), I would think that you normally wouldn't need a map the entire way. Just a cached map of your destination where you're no longer traveling several miles on the same highway or county road.
And too, with the ability to save 10 maps, that's 4000sq/miles of map you can utilize. That would easily accommodate a cross country trip across the US if you just cached the areas of turn off points and your final destination.
