Just a thought,
They look like they're about the size of FG preview blocks, I'm wondering if those grey rectangles are the FG blocks that ran out of Refraction depth...
As in, the level of refractions (set in the "Raytracing" tab of the "Render Settings" when Mental Ray is selected.) is not high enough, and after the maximum number have been calculated all light calcs stop for that area, and you end up with just the ambient light (or a light that doesn't pass through anything) illuminating those spots.
EDIT: Just had another look at your image and noticed that there are in fact caustics and shadows shown on the defective area of the image.
It just reminded me of something that I have that occasionally pops up. I get horizontal lines across my image. It's not a solid color, but rather the whole scanline get shifted 5 or so pixels to the right. Only happens when I use Mental Ray. Only happens with some images. There are times when the same scene will display this artifact from only some angles!!!
I never bother to read the threads in their entirety, but from what I've skimmed around the forums, I gather that MR still has a few bugs here and there. If you can, perhaps upgrading MR would help..