Now what WOULD be awesome would be coupling the Arduino to an actual gear changer.. Some steppers/servos attached to the gear switch cables would do magic
The only issue is that if you have ever tried to change gear by pulling the brake cable (perhaps when you have been building a bike) you will know that it takes a lot of force and you would need a really high torque servo/stepper. It would be cool though.
It would be even better with the NuVinci CVT hub gear because then it wouldn't have to 'change gear', just turn it up or down...
(http://www.fallbrooktech.com/Nuvinci.asp)
(http://www.jensonusa.com/store/product/HU299A08-Nuvinci+Cvp+Rear+Hub.aspx?sc=FRGLUK&cc=GBP)
Mowcius