mfb said:
It is complicated
"Metal" is not interesting - I guess you mean "ferromagnetic" (only some materials are ferromagnetic, iron is the most relevant example)
I think it's even more complicated than that. That link is about forces between magnets.
I just did an experiment with my 1 Tesla rare Earth magnets and a pair of nails.
The nails weigh about 1/2 gram, are 1.5 cm long, 2 mm in diameter, and were originally not magnetized.
The magnet is a smooth edged cube, the edges measuring roughly 4.1 mm.
The magnet is able to lift a nail off the table from a distance of 1 cm.
The magnet nail combination was not able to life the 2nd nail from the table until the distance was ≈1 millimeter.
When the magnet was removed from the first nail, the nails stayed attached. I had created magnets!
Trying to determine the strength of the residual nail flux density, I was only able to determine that a separation of 0.1 mm resulted in nail #1 not being able to support nail #2.
The last measurement I did, was to remove the magnet, flip the poles, and slowly bring it towards the nails. When the magnet was 2.5 cm from nail #1, nail #2 was released. I'm guessing that the field strength of the two nails can be deduced from this measurement. (Perhaps I should turn this problem over to micromass, for another "Math Challenge"

)
The nails were still both magnetized after this portion of the experiment, as each could support the others weight.
But introducing unmagnetized nail #3, neither was able to budge it.
Anyways, the problem with this problem, as I see it, is that the magnetization of the ferromagnetic material is influenced, and changed by the permanent magnets, making this a really dynamic problem. If I flip the poles of the permanent magnet, and bring it to the two nails, their magnetic fields reverse.
Problems with this experiment:
Like many nails, these had flat heads and pointy tails. Geometry is probably critical.
When I find my dremel tool, I'll redo the experiment.