I saw a couple last summer on the Camino Primativo on the part you would have hoped was the least likely to see them - on the wonderful isolated part from Borres to Berducedo that passes via los Hospitales. There were two of them and of course, the sound they made was so out of place in the silence. I did just wonder though, if they were not actually using the path as a route to go to work, rather than for a day biking.
I saw and heard a couple of Quand bikes on the Chemin du Puy near Saugues and then I must admit my feelings were then not quite as forgiving - my walking compnions and I were hoping the Bête du Gévaudan would make a brief return! (If you have yet to walk from Le Puy, at the entry to Saugues, there is a huge wooden statue commemorating this mythical beast that was blamed for killing 100+ people in the 18th century.)
And this is the result of what motobikes can do to the Camino. This was taken about 6km before Sauveterre de Béarn, a day and a half before SJPdP in March 2008. So let us hope that bikes will keep to the roads where it is not for us to walk, and let us walk where it is not for the bikes to ride