My engine oil plug is not magnetic, but my manual tranny plug is.
Certainly collects significant filings. I assumed this was the stock plug, but perhaps not.
I considered the epoxy-a-magnet method as well, but after playing around with some rare-earth magnets, I changed my mind. These things are amazingly powerful. You can place one on the back of your hand, and another on your palm, and they will hold firm. Just keep them away from your CRTs and credit cards!
I stuck a small rare earth magnet to the outside end of the engine oil plug, and upon my next oil change, filings had indeed stuck to the plug. I got the magnets at http://www.leevalley.com
Put one on my diff too. We'll see if it gets anything.
I have seen those magnetic 'bracelets' for oil filters. They probably work as well, though how would you know? I suppose you could stick a rare earth on the filter as well (cheaper than the bracelet anyhow).
Best of luck.