You didn't say how big the copter is, but you might take a look at the JeeNode. It's
about as small as a 28 pin DIP Arduino board can be. Runs off 3.3V v.reg, and has
an RF transceiver so you can talk to it. Would need a 3.3V IMU.
Hi nice to know i have a colleague, i'm trying to build a quadrotor with just an Arduino UNO and an old 4ch RC set, i've got the first parts of the hard job almost done, i connected the RC with the UNO and managet to work them together then i wrote the skeleton for the software and a basic open loop trial version
i'm starting the prototyping with a small quadcopter but the control interface i have designed will surely suit on a hely
in my plans there are an octocopter with rotor redundancy and a gas powered hely with on board generator and a small 2S LiPofor backup power, just to fill the tank and fly for hours