How can I build my own open-source cyclometer?
The most detailed account/details I've come across for building your own bike computer is here on PIClist.

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.