Also, most of the navigation and map programs need the internet to display the map. If you are out of 3G coverage you may end up with a nice blue dot on a blank page. If you don't want to buy one of the expensive apps with built in maps, I can think of two options.
Motion X GPS, or Motion X GPS Drive. Both of these have a history buffer for maps that can be set up to 2GB. Other apps also have buffers, but I don't know how big they are. If you review the area in the map while you have internet you should have maps. This is a bit chancy,
So my second choice is Topo Maps for iPad. While the app costs money you can download topographical maps from just about anywhere in the U.S. for free. It's a good choice for hunting and fishing, at least I think so.
Be sure to download the maps you want before you lose cell coverage.
Edit: Just looked up Navfree because Dont suggested it, and always has good advice. It says it keeps local maps, and since it's free I'd call it an excellent choice. I still think you'd like Topo, if you don't mind spending the money.