Garet, I've never heard of that. I'd think any pieces of metal in the oil would be magnetized from friction. Then they'd be attracted to the oil pan anyway. If they were attached to other parts like pistons or cylinder walls, a magnet wouldn't dislodge them (maybe, if it had the power of those used in a junkyard).
They got them, and they work. The best way to go, I think, is to get a drain plug with the magnet built in. It catches small flakes of metal. Thing is, if you got metal shavings in the pan, you most likely have bigger problems!
Magnets are common in racing engines since the oil filtres we use are not capable of trapping particles as small as you would need in a "production" engine. We would prefer to screen out just large chunks and NOT have the filtre plug up, thereby increasing our chances of finishing the race. You can't win if you don't finish, so we'll use magnets in various places to stop any broken pieces from causing catastrophic failures.
For example, we will epoxy magnets near the oil drainback holes in the cylinder heads and in the lifter valley. We also epoxy stainless steel screens over the holes in the lifter valley to prevent broken pieces from falling into contact with the camshaft and/or lifters.
I also run a magnetic drainplug, and not just in my race car, but in all my engines. I don't put magnets inside any of my oil pans because that would require removing the oil pan to clean/inspect the magnets. (Keep in mind that in my race car, I change the oil after every race weekend. Oil is cheap; engines are not!)
I also remember a magnet that used to be available from an aftermarket company that was curved to fit the outside of oil filtres. I have not seen this product advertised for a couple of years, so I don't know if it was ineffective, or just didn't sell. Perhaps check out E-Bay or do a search on google.com?
To address Mick's comments, I have never seen metallic particles get magnetized by friction. If you doubt me, just run a magnet around in your oil drain pan when you change your oil. I can almost guarantee that there will be some very small particles suspended in the oil (it will almost look like tiny traces of silver paint or something like that). If even these small particles are not magnetized enough to "stick to the pan," you can bet that NONE of them are magnetized.
In addition to MTCK's comments, a small amount of metallic particles is normal in the oil. It's unavoidable, since everything inside the engine will wear just a bit.
When I change my oil in my cars or trucks, there is always a little bit of "snowflake-looking" metal sticking to my magnetic drain plug. If any pieces are 1/32" or larger (and that's damn small), then yes, you probably have abnormal wear.
If anyone wants to scare themselves, do what we racers do: cut open your oil filtre and look at the crap trapped inside of there! If there was ever any doubt that they work, it will be apparent in the folds of the cardboard.
My motorcycle has magnetic drain plugs for the engine oil and final drive oil, and they do pick stuff up!
Doing an oil change tonight, and although there aren't any big chunks of metal there (thank goodness!) there are the "snowflakes" that raceman mentions.
There's also a small square magnet that sits in a groove cast into the inside of the left engine cover. Since mine's an '83 Virago (the ones where they DIDN'T quite the starter drive design right) I figure the factory put it there to collect bits of starter gear tooth.......................