Well, I tried increasing the refraction quality, and it was still there. The value started at 0.2, and I raised it to 2 and 20 with no result. (I also tried playing with some of the other settings, no help.) It's there whether shadows are set to raytrace or shadow map or off completely (at the light or in the render settings box.) Here are the material settings, shown with the refraction quality turned way up, and the result.

I can't have easy problems, can I? ;)