thanks LauriePriest - yeah - i've been using a 32 bit tiff from zbrush.
i'm pretty sure that the normal map is fine - i can see it in the viewport and in zmapper. i think mental ray is just rendering my normal map as a bump map cos when i used the normal map as a bump, it renders the same.
so i'm not sure if there's something wrong with my bump2dnode. is there something i have to switch on to turn on tangent normals?
i'm wondering if it's a problem in maya 7 - i've seen a few people with the same problem.
i had a look for the gamma offset value in zbrush, but didn't have any luck.