AFAIK, normal mapping only works where the diffuse component of the light, the one that caries the bump effect is not zero, so it doesn't work on polygons that are facing away or are almost facing away from the light source.

You can use ambient bump mapping approximation or spherical harmonics that sample the normal map, but I don't think this is necessary for terrain. In most of the cases, you won't have faces are perpendicular on the direction of the light and you will have a non-zero diffuse contribution as long as the light source is not very low.