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.