How about adding ultrasonic sensors, wireless cameras, and an XBee wireless link? I got an ArduPilot with GPS for Christmas, so we’ll see what happens there. I also plan to connect an electric motor to the lawn mower drive shaft to charge the batteries, which will also act as an onboard electric starter for the engine in case it dies during operation.
To automate the process, I’d start by mowing the grass with the R/C remote while using a GPS logger to record its movements. Then the ArduPilot would guide the Lawnbot through the recorded GPS path, using sensors to keep it from hitting anything the GPS didn’t catch. Of course, I’d be inside, watching via camera while enjoying a cold beverage.