I can only get it to work with MentalRay, I've no idea if that's the way it is or if I'm missing something. Ha, ha I read a post on this a while ago - thought I'd need it some day.. Though it'd be nice if I could remember things I know I need.

It's just a matter of the shader.

I just created a poly plane and a sphere. I've assigned a Blinn to the plane and set the color slider all the way to the left, giving me a black Material Sample. I then cranked the reflectivity up to full, rendered it with Mental Ray and it worked ok.

Obviously, you'll want to ad some lights to a proper scene as will you probably want to tweak the settings for the shader a little more, but it works.

S.