Michiel has a build log of his mini R2D2 project over at letsmakerobots and its turning out really awesome. I’ve seen the hard core guys who build full scale replicas but never really thought of building a miniature. He’s broken the build out into sections like Dome, Body, Legs, etc.
He doesn’t have the electronics section up but he is using a Raspberry Pi and a Picaxe micro controller. I would guess the Pi handles the main control paths and the Picaxe doing most of the sensor feedback, servos and leds.