Good to know. My sales route has some dead spots here in northern, PA near Towanda and Montrose. Thanks to the OP for the question as it's one I wanted to know myself.
I actually started the GPS NAV in the middle of the dead zone and last week a few days after I got my phone looking for some shortcuts to in a territory in which I only know the basic route for...and I only got the blue dot/arrow on a blank screen with no maps not knowing you needed a cell service to get the maps. Even though I know where I was and going, this is a feature I hope that it' capable of. Since that day, I have downloaded a few GPS apps for geocaching and entertainment purposes and will test them out too. I am assuming it till tell me my coordinates and direction heading without cell service.
Perhaps next Tuesday I will start the GPS NAV in the 3G zone and take it into the dead/no coverage area of my route and see if you guys are right. It does seem to make sense that it would based on what I have found from playing aroung with it thus far.