Flatlander, The iPhone 4 series units have a dedicated GPS chip which do not require cell service as long as it is not in Airplane mode as Pueblo stated you are good to go. I just used it for 4 scouting days in the San Juans last week and it worked perfectly. Like Pueblo I find it's pretty rare i need it but it's nice to have when I do. I also find the GPS to be just as accurate as any garmin i have seen as well. On a side note regarding any of the topo map applications, I also had a Nat Geo Trails Illustrated map with me and for the area I was in the Nat Geo map was very accurate with refelcting new trails that that current USGS 1:24,000 scale maps did not show. Without the updated Nat Geo map I would have humped an extra 6 miles to complete the 34 mile loop. I sure do want to know where every "backcountry highway" is when i need it and that is a perfect example of why having redundancy in your nav tools is a good thing.......