Is there an official cycles material preview ball to compare the shader with 100% white? I could only find the following image here. Except of the inner material this looks like what I'm looking for:
Image by David Zerba
The Blender Material Preview Scene (BMPS) was included in the 2.61 demo files, which you can download here:
Just open the bmps.blend file to find the material shader ball scene.
There is also a material preview scene provided by blenderdiplom.com with a better light setup: