We are a small team of developers in Arizona working on a lightweight autopilot for RC planes. The program works with the Mongoose IMU (http://"http://store.ckdevices.com/products/Mongoose-9DoF-IMU-with-Barometric-Pressure-Sensor-.html") and the Arduino Nano.
You can follow our progress here: http://radarproject.wordpress.com (http://radarproject.wordpress.com)
I have a similar goal. There is a thread going under Project Guidance. I'm using the KK board to keep the plane stable. In the end I will use a barometer and compass along with GPS to get me to my destination. First step is altitude hold. I'm testing this with a Quadcopter. A plane will be much easier when I'm done.
Ah, what is a KK board?
I haven't gotten to the altitude hold yet. I'm just working on (Almost finished) a fly loop which keeps the plane at the desired roll and pitch for the desired amount of time. I haven't figured out the best way to correct altitude with the pitch correction as well. I'll have to think about that very soon.