It would be good to have a method to rotate cursor movement, per
mouse. This would be useful for contoured or handheld trackballs, where
there is a natural and fairly constant rotational offset between the
tracking sensors and the actual movement.

For instance: I have a handheld trackball (from CompUSA) where the
natrual motion of my thumb rolling the ball is (estimated) 35 degrees
anticlockwise from 'true', when I'm holding it comfortably in my hand.