Has this gone any further? It should be possable using the fm transmitter and reciver on the n900 as you can find some simple metal detector designs on the net one is as simple as taping a calculator to an am radio (am though not sure if it has to be)

Surely the interferance from the radio could be used as a measurement? I have noticed when driving my car that whenever I drive close to large metal objects it gets worse most notaceable is water of couurse, I get massive interferance driving past the public swimming pool!