Funny you bring this up. I thought I was an original thinker..
I had an idea similar to this a while back. I wanted to make an open source inertial navigation system for FRC with focus on ease of use and availability of parts. INS's are tough, as you need to have pretty custom hardware including the correct sensory and the horse power to process your signals. So, I figured I could "borrow" and hack up hardware thats available in the market place. Also, I wanted this system to be able to handle other robot functions, such as autonomous control, so it needed to have a screen.
I stumbled upon
this., which gave me means to get pretty accurate 3 axis acceleration and yaw rate readings on the nintendo DS. I looked into the DS linux and homebrew projects, along with how to grab hold of an RS-232 serial connection through the SPI port it uses to read games. Running Linux would be my first choice on this, as I could write all of the software with little overhead and its just plain cool.
So, the DS enables you to have an INS with touchscreen for a pretty good price. Unfortunately I haven't had time/money/motivation to follow up on this. Oh well, maybe next year.