Here you see the beginnings of Seacharger's autopilot. Clockwise from the upper left: tilt-compensated compass (Devantech CMPS-11), Seeeduino Mega (basically an Arduino Mega in a slightly smaller form factor), servo mux board from Pololu, R/C receiver, Adafruit GPS, and a servo. Special thanks to JT for putting together all these components and getting them to talk to each other!
After reading about Arduino code and making a mockup of the autopilot in Excel, I managed to get a pretty decent autopilot working. What I mean by this is that I could program in some waypoints and then take this setup to a local park and have it guide me to the waypoints by moving the servo arm in the direction it wanted me to walk. There were a lot of bugs in the code, mainly having to do with passing from 359 degrees heading to 0 degrees heading or from 180 degrees to -180 degrees. But I got all those worked out and it's all good now. This kind of work is really fun because you can make progress so quickly. Those software guys have it easy!