I think that is a pretty challenging project. You'll need a controller (microprocessor??) to take in feedback signals from an accelerometer and gyro and then adjust the speed of hte motors accordingly (otherwise it wont be stable in the least). I would imagine that this processor would do the mixing for you. I know this because i have considered doing a project like this, and then ditched it because it'd be too difficult for what i have time for. If you are going to continue on this project, Microchip makes some really cool and inexpensive processors you may want to consider using. Good Luck! Make sure you post pics if you get it going!

-Dan