Check out the "terraplane" design from David Kirk:



Here's what he says about it:

"The curved seatstays of the Terraplane allow a small amount of vertical movement and are designed to keep the rear tire in firm and constant contact with the surface. Each set of stays is bent by hand to a given radius determined by the size and weight of the rider and the intended use of the bike. The feel and stiffness of the frame is regulated by the amount of bend in the stays. Heavy riders generally would have less bend to allow less movement. Lighter riders would have more bend and more travel."