Todays century ride cyclometer vs GPSr measurements:

GPS 1 (eTrex Vista, in the aero bars): 160.14km (99.50 mi)
GPS 2 (GPSMAP 60CS, on the camelbak with external antenna): 160km ( didn't turn the tracklogging on - woops, but the trip odometer shows an even 160km, can't get the decimal point without the tracklog )
Cateye Astrale 8 cyclometer: 162.85 km (101.19 mi)

Considering the cyclometer takes distance based on every rotation of the wheel, while 655 of 2235 trackpoints on the eTrex were over 100 meters away from their previous trackpoint, I'd say the cyclometer was probably spot on for the distance of wheel revolutions. Good lord knows I don't ride any 100 meter stretch in a perfectly straight line - and some of the track points were 270 meters apart.

Meanwhile, I checked my turn around point on my "point and click" route to my actual turn around point (when I hit the 80km mark on the GPSr), and the two points were roughly .2 km difference. The actual turn around was .2km shorter than the turn around that I "point and clicked" on the map.