Hi, if you're only using mental ray and FG, you probably just need to go to your camera attributes and change the environment color from black to white. Just guessing though...I don't see any reason for it not to work like you have it though.

edit: I just tried with both white and black environment and it works both ways. If you can't figure it out, might want to do like nextdesign said and send him the file to look at for you.