Look at this:
Nokia N900 Becomes A Bike Dashboard
Bike Dashboard is a N900 device turned bicycle computer mounted on the handlebar of a bike with a number of sensors and features such as displaying speed, mapped routes with GPS, distance traveled, bike’s light control and even a horn.
To get more details, see:
http://www.psfk.com/2010/04/nokia-n900-becomes-a-bike-dashboard.html