Fresnel + Shadow = Abrupt Cutoff (Bug?)

Some Time ago I started using Blender’s Fresnel Material to imitate smoother Shading or Translucency in GLSL Mode, but the Problem I have is about some weird Material Behavior: If there is a Light shining onto the Surface with Shadows activated, the one half of the Fresnel Shader that is on the dark Side turns black abruptly, taking away all the Smoothness (that also occurs when I use Variance Shadows, which are very soft).
I wonder if this could be a Bug, because it looks rather wrong.

Here is a Screenshot:

(I have posted this before in the Material Subforum, but nobody cared – I guess it is too bge-related…)