Here's what I mean by blurring an object into another. For the ball that's partially stuck into the plane, you can see the edge is a little bit disturbed because the plane has been blurred "into" the sphere. The blur effect overlaps the edge of the sphere a little bit. This can be much more obvious when you turn the blur up to large values. For the ball that's just sitting on the plane, the contact point is hidden enough from the viewer so as to not be an issue. Either way, this is a fantastic trick! -Kix

-Kix