I have noticed something really weird… On a model I loaded (and perhaps the whole scene?) the lighting seems to be affected by the orientation of the Camera!!!
I show you three screen shots taken exactly from the same position, except facing in three different directions:
The model is an imported OBJ model. The model itself does not have any light associated with it… Actually it looks too dark to be right. Before, the model had a light on its own, and it looked awful (one side completely illuminated without any smooth shadow cue, and the other simply black).
What can it be? I know it is not an update render state not geometric state, because I update those.
Check your material states. Some 3d tools do not export material properties correctly.
You are extremely right! The diffuse color was set to 0, and the specular to 0.5… Now, it does make sense for the specular color to vary with camera orientation right? that is what it is supposed to do! Mhmm… Hate exporters/importers… everything should use jME's native format 
(I know this is ridiculous )