The garmin Forerunner 101 can function as a cyclometer and has a simple track back feature. I've used one on my folding bike for years. Another Garmin I've heard good things about is the "Gecko".

I have this model...

http://catalog.ebay.com/Garmin-Forer...d=p3286.c0.m14

Here is a pic of it on one of my folders...

http://s164.photobucket.com/albums/u...t=DSC01999.jpg