For anybody using an iPhone it's important to read the battery conservation section in the article: Instructions for using iPhone as Backpacking GPS/Mapping device.
The iPhone does not have removable battery, as others have said. However, there are some important things you can do to preserve battery, and there are small light external battery rechargers. Using the battery conservation measures, I've used mine for 4 day stretches without recharge, and AlanD has used his on 6 day trips.
The mapping/GPS apps for the iPhone are great. I've used it is my primary navigation tool on three six-week trips in the past year to places where good paper maps are either unavailable or super expensive (Turkey, Australia, Spain). For a $10, you get track and waypoint management, downloadable maps and satellite imagery of the world, etc etc. If you've got an iPhone (or Android too), then it's worth the investment to give it a try.
Good luck, AmyL