From suite101.com:

One of the problems with using an Android-based smartphone such as the Droid, Droid Eris, or Droid X as a hiker’s GPS is that maps don’t work when there is no cell coverage, which is common in areas where people like to hike or bike recreationally.

Google Maps will pre-cache some maps but it’s unpredictable as to whether it will fetch the right maps or delete the cached maps altogether. The real solution is to use an offline GPS program instead of the Google Maps program, which still requires access to the Internet

…

For those willing to put the effort, a free program called OruxMaps can make use of GPS maps stored on a Android smartphone’s SD card. The process of getting the maps is actually very straightforward by using another program. For most people, it should take no more than a few minutes to begin downloading the correct maps for use on an Android device with just GPS signals.

Very cool stuff.  I’m going to have to check this out for the next time we head to Vermont.