Sehr interessant, Wolfgang.
My sense is that the French cling a little more strongly to the old ways: I don't think the "common fate" rule works there. You would have to be trapped in the elevator a long time to call someone you hadn't met before "tu". Nor would I go to "tu" at a party, even if I use that form with the host. But as I said, students (and musicians, artists, etc.) are all "tu" to one another. I have even been in a class at the Alliance Française in Paris where the instructor was called "tu" by the students -- perhaps one of those strange French gestures towards republicanism -- or the students didn't know any better.
Members of the French political and journalism set are always tickled to hear bigwigs like Chirac and Jospin "tutoyer" one another. Like almost every member of the upper political echelons, they are "Énarques" -- graduates of the elite ÉNA school of government, and by tradition they are always "tu" to one another.
Once, in a store in Switzerland, "du hast recht" slipped out from *my* student days (und dass war vor 40 jahre). I immediately corrected myself, but the young salesman didn't mind in the least.