Humm...
Sorry, it seems i've confused myself once more. 
This page lists the parts used for a quadcopter build aeroquad.info on which it states that they used a 5 DOF IMU (Inertial Measurement Unit) and also a Dual Axis Gyroscope which apparently "adds the 6th DOF"
Unfortunately, the site neglects to mention the actual build process (even though there is a build process page) although I did manage to find a photo in which you can clearly see that some of the components are mounted vertically Attachment browser: ufo03.jpg by joeling - RC Groups
Am I correct in assuming that the 5 DOF IMU (accelerometer and gyroscope) is mounted horizontally to measure acceleration along all 3 axis and rotation around the X and Z axis?
Bearing that in mind, what purpose does the Dual Axis Gyroscope serve? Even mounted vertically, surely it would still measure rotations around the X and Z axis?
Does this mean that in order to determine rotations around the Y axis, the only component capable of such would be a compass sensor or magnetometer? Given that I want to create an autonomous vehicle, the orientation of the device is crucial seeing as it won't be me who's flying it and therefore knowledgeable as to which way it's facing.
Basically, I need to measure rotation around the X and Z axis to determine whether the tri-copter is stable. Currently, something that hovers will suffice until I think about adding in orientation/heading and waypoint finding at a later date. Which sensor would provide the appropriate feedback to achieve this? I'm so confused right now :
Thanks for reading, any help is appreciated!