This previous answer does not pertain to Unity Terrains??? or Yes?

Unity now supports procedural materials by importing allegorithmic substance materials

Substances will be available on the asset store amongst other places and you'll be able to:

• Tweak the exposed parameters of the material e.g. tiling, colours etc
• Preview in scene with bump mapping, specular etc.
• Create multiple materials per material asset with different paramaters

Applying Substance material:

• Drag the Material (sphere icon within the the Material Asset) on to the object or apply as you would a regular material

If you are importing an asset with substance already applied e.g. from Max or Maya you will need to re-apply substance material in Unity after importing, as currently you cannot import substances and settings from Max/Maya. You can place your substances from your library or those bundled with your art package directly in your unity project.