PhoneDrone is an Arduino compatible interface board that you can connect with any Android device (2.3.4 or higher), allowing you to use or phone to communicate with the RC/UAV world. THe board features 8 PWM inputs, and 8 PWM outputs, as well as PWM-to-PPM conversion and multiplexing between your RC gear and Android device.
For UAV applications you could use your Android phone as a long range wireless communications device instead of the more traditional data modems which have complexities due to interference between the modem and your RC gear. Alternatively you could use your ArduPilot Mega autopilot for aircraft control, and use your Android device to run complex navigation, path planning, or evin image processing algorithms (object tracking) and sending mission commands back to APM. The limit is only your imagination.