Beasty, nice name, and thanks for the lead! This setting is controlled in the material's 'Maps' settings; where the mesh's size relative to an image pixel can be set. I changed it from 4 to 1 (1 mesh poly per pixel) and it did indeed change the effect.
So, I'm not yet getting smooth siding material, and after a quick search on these forums I see the general consensus is siding should be modeled with geometry. This makes sense because it's been painfully hard to get good results by using this displacement method.