Following up on a discussion we were having at the event today, I've found how to change the icons displayed on my Garmin GPSMap 60c (and this procedure will probably work on all other GPSr's that display icons for waypoints).
Thanks to Mainiac1957 for getting me almost there.
Change the icons in GSAK. Clicking on GPS/send waypoints. On the next screen, you'll see a list of supported icons on the left. Clicking "change" will alllow you to specify the icons used.