A while back, a friend of ours asked if we could build one these for him:
Since I like a challenge, I went for it. I ended up using the EM-406A GPS module from U.S. Globalsat, a standard Arduino 328 board, a backlit 16×2 character display from Futurlec.com, and the Pololu soft power switch from Sparkfun.com. The code will be cleaned up and posted soon.
Here are some pics of the build and completed project:
Original post: http://blog.shop.23b.org/2010/04/reverse-geocache.html