Interesting. The Lee Valley site mentions "magnet cups" as a method for "focusing" the magnetic field:

"When a magnet is placed in a ferromagnetic cup (Figure 3), the cup further magnifies the effect by eliminating the air gap (air is a poor conductor of magnetic fields) and brings both poles of the magnet to grip on the same surface. This is similar in principle to a horseshoe magnet. A rare-earth magnet in a steel cup provides four times the strength of a bare magnet. A cup provides a disc magnet the optimal magnetic flux focus into the smallest gap area." (http://www.leevalley.com/hardware/page.asp?page=40077&category=3,42363&ccurrency=2&SID=)

Have your tried these steel cups? It seems to this nonphysicist that a magnet-to-magnet setup would double the attractive force, while the steel cup-to-magnet would quadruple it, if the Lee Valley claim is correct. Intuitively it would appear that either setup would follow the inverse square law of force vs. distance, but maybe you have some experimental results otherwise?