I have played Penumbra: Overture perfectly on my GF6600 video card, but then I bougth new radeon HD X2600pro and countinued playing.
Now I can't complete the last level because of the graphical bug.
I have noticed, that textures become normal when i put some light on them(but not the ground ones).
I think that the problem is in shaders(the ATI video cards have different shader commands the the Nvidia ones)
Maybe it might help you to solve this problem
P.s. And thank you for the fast reply, I see that you take good care of your customers