You can have shadows on terrain. A simple way is a material with a very low diffuse and ambient value.

The most powerful way is the shader that you can find in the template folder. This shader can paint several layers of tileable textures and one layer of shadows over each other. With this you can paint your shadows or you can bake the shadow in an external software (like FreeWorld3d). So every tree could cast a shadow. But this would be a static solution.