I've been bumped a few times between Seattle and Phoenix during the winter months on Alaska Airlines. Friday late afternoon flights between Seattle and Detroit used to offer bumping opportunity until NW added DC-10's to the route.
I have never been involuntarily bumped but one time I was flying between Seattle and Eugene, OR on Horizon Airlines and the flight attendant announced that the plane had exceeded its weight limit! Four people including myself took a flight 60 minutes later and were compensated with a free ticket.