I just returned from a 2 week trip in the pyrenees (both French as Spanish). Camped in the wild every night, no problem. Only incident was a forest guard that told us it was illegal to take a car outside the paved roads, he said it was no problem to camp though, as long as we would leave the car on some public road (he really meant paved). This was in a forest that was classified as a "reserve natural".
Basicly the French side of the Pyrenees is much stricter then the Spanish side. A lot of dust roads in France are no-go-area for 4x4's, in Spain they put a sign to recommend a 4x4 :-) .
Farmers are generally friendly too, just ask them if you can camp in one of their fields.
Just use your common sense and you'll be fine...