What does selecting "Normal map" in the image sampling panel actually do? Whenever I import an image in Blender to be used as a normal map I have to select "Normal map" in "Image Sampling" under "Texture". I remember that this was not always the case, at least I don't remember having to do that in older versions (aroung 8 years ago) though I could be misremembering. I'm also asking because as a programmer I've used normal maps in OpenGL and they went through the usual texture pipeline, no special treatment needed. So I wonder what Blender is doing with this information.
Assuming you're using the old Blender Render engine, this checkbox just tells Blender that the texture in question is a normal map rather than a bump map so that it knows which shader to use.