SparkFun just released an Arduino-compatible autopilot board designed by Chris Anderson and Jordi Muñoz of DIY Drones. I checked out Chris' board a while back and was under the impression that it was still under construction, though if SparkFun is manufacturing the board now it must be ready for the average hobbist.
For $85 you can have the brains and GPS for your own UAV.
Personally I've always wanted to create a boat UAV that autonomously roves the coast traveling to far away places with a cellular modem reporting back with pictures and charging with solar panels.
http://www.sparkfun.com/commerce/product_info.php?products_id=8785