Dave Mabe has published a couple of articles on compiling race maps by taking GPS tracklogs, processing them, and running the resulting data file through the Google Maps API. The articles are:
Looks like he's getting a lot of ideas from the Mapping Hacks book published by O'Reilly. I saw this book in a local Barnes & Noble the other day and I definitely want a copy. I just don't have time to work through all the interesting recipies in it right now.