The most detailed account/details I've come across for building your own bike computer is here on PIClist: http://www.piclist.com/techref/piclist/biketut/index.htmPIClist.
Those pages contain detailed instructions for building a bike computer using a PIC microcontroller linked to a Hall Effect Sensor (it detects the magnet attached to a spoke as the wheel spins). In addition to the build details it also features a detailed discussion of the design process which the creator undertook whilst creating the computer.
