Jim Miller, who posts here as HWPSlotCars, has recently added a magnet chassis to his Shapeways '3D printing' store.
This version has what I dubbed Infinitely adjustable magnets (IAM) as the magnets can be moved up and down in the chassis with an allen key.
Jim and I have been working on this chassis for a couple of years, following on from an idea I had in 1993, and executed on two prototypes that never went anywhere.
Eagle racing also has a chassis that has IAM, made one at a time and featuring a cool carbon fibre chassis :)
Maybe IAM's time has finally come?