Also remember that special shaders are used when creating Terrain, Terraingen for example will create a special terrain shader just for your terrain automaticly, that shader will have blending in it and is also a 'phong shader' which uses the light to create the illusion of smoother edges ('phong' is a common term in 3d graphics where the harsh edges of triangles are lighted to create the illusion of smoothness) .
How does all this apply? Well you can't just stick another brush with no phong shader in your terrain because it will have harsh shadows where the terrain does not (unless its a wall or something), and if you want the brush (for a ledge lets say) to look similar or just like your terrain you will need to create a phong shader for it, and possibly some blending (if you wanted to go that far) . I have just discovered all of this after much consternation. Echoman gave you good links,
here is the main page for those articles, check out phong shaders, rockwall corridor, rockwall detail, and terrain blending to get the complete picture.
http://www.planetquake.com/simland/pages/articles.htm
