Metal Detect is an iPhone application which turns the iPhone into a metal detector by using the iPhone's magnetometer. It's impressively accurate. Update: Now has been approved. Look for it in the App Store. Application name: Metal Detect wevito.com
This is a one kind detector built by my friend 12 years ago from some parts in a junk box. He had about $35.00 total in it. It is not for sale, rent or lease. He doesn't even remember what he did.