Thanks for an interesting thread, Brunetterr, I hope you stay around for a long while.

To me, as a Brit, "to quieten" comes much more naturally as a verb, although I'll agree with b-y that "to quiet" is in very common use over here these days. But whenever I hear it, my mind registers it as 'an Amercanism' (with all the pejorative connotations that phrase has over here!)

As to "quieten down!", used as an imperative, I greatly prefer the old-fashioned (and, these days, rarely heard) Brit. naval command, "Keep silence!"