Mark,
It is the UB version. I move the same exact file to the iMac and in a portion of the rendering, it places arbitrary pixels. The funny thing is that it always places them in the same exact portion of the model, but not in every instance of it. In other words I have a shape that is repeated 3 times on the image, it miss-renders on two of them and this only happens exclusively when I render on the Intel Mac. Every other aspect about the software seems to work very well. On a G5 it works flawlessly.

What hardware are you running Carrara 6 on? I am getting a MacPro later this year and I would like to know if Carrara runs well on it.