I've heard of a COFI mortgage, where the finance charges fluctuate depending on the year, etc.
We had a COFI mortgage a few years ago on our first house - before the rates came way down. It was on some kind of sliding scale...one year you pay 6%, the next 6.2%, the next 5.9%...it's a good mortgage to have if you don't want to mess with ever refinancing. I wish I knew more about it, but try COFI in google.