Just my 2 cents on this. Don't use the "ground plane" at all. I've always found that it doesn't corectly render shadows. This may have been fixed in the most recent version, but I still keep away from the "ground plane" as a rule anyway. Remember, this is just MY opinion. I use one of the many cubes and planes in the props section to make a quick plane. You can better apply textures and they cast/receive shadows far better than the "ground plane". I also create my own floors and such. A very easy thing to do and you have exactly what you want and do not have to rely on the quirkyness of the "ground plane". James