In 1999, Del Doc, for the ATC, did a GPS survey of the AT. Lots of us ran into him.
The recent release of the AT maps on CD-ROM also allow for waypoints and trailmaps to be downloaded into a GPS unit which could be helpful to you. It now makes the idea of using a GPS on the trail maybe a useful idea if one needs the co-ordinate system used by a GPS and 7.5 degree maps to establish rendez-vous points for pickups and/or locations of cached food for a hiker left by a support team. Or it could be used to blaze a wilderness trail and return to the AT if one wants to explore unmarked trail just for fun. Wilderness areas on the trail are notorious for being poorly marked. And side trails off of the AT in these areas might be more poorly marked than the AT.
Otherwise, the trail is so well white blazed, a GPS seems hardly worth the extra weight in one’s pack. But it is a great toy. And we are guys after all! No need to ask directions with this toy, now is there.
Mr. Boo