During my brief stint with Lockheed in Minneapolis, I contributed a bit of code to an after-hours UAV project. Unfortunately I didn't get involved until almost the end of my time up there, but I was enthralled with the project. Since then I've always had an idea in the back of my mind to buy an off-the-shelf RC plane and add an autopilot to it.
The project either boils down to a ton of time or a ton of money. That might be changing though. Some clever individual has created a simple autopilot with pieces from the Lego Mindstorms kit. The kit now includes a gyro sensor and that is the piece that enabled this project. Currently the autopilot will maintain level flight and control the rudder to return the plane to the launch area.
If you see a tiny plane flying overhead taking pictures of you, look for the Studio711 logo on it. I'd love to create my own version of this project.