Well, most things will have some non-zero magnetic susceptibility that will affect an ambient magnetic field. Ferromagnetic materials will have a larger effect.

Well, most things will have some non-zero magnetic susceptibility that will affect an ambient magnetic field. Ferromagnetic materials will have a larger effect. I just loaded "Metal Detector" on my Nexus One and played around with it. It seemed to only detect ferrous metals. For example it picked small steel tools fine, but couldn't sense an entire plastic bag full of (US) coins or aluminum objects.