While watching this weeks episode, my wife may have caught a mistake made by the writers. She works in MRI using a giant electromagnet and said fillings aren't magnetic. It could also be a clue to the nature of the anomaly. It may be more than just a magnet. It could possibly attract conductive metal molecules or change its molecular structure. Seems to be a stretch, but you never know with Lost. They have emphasized special energy filled places. i.e. Claire and Rose's Psychic along with Mrs. Hawking's reference to these at the Lantern. The anomaly could be something more than just a magnetic pocket.