This idea was great when bumpers were METAL.
Now all this crap of plastic and fiberglass - it has to be a very strong magnet to "feel" the steel way underneath past a layer of air, too.
The really sad thing is all this stuff is painted along w/the rest of the body. You are really screwed when you take off a bad bumper sticker. My car is all scuffed up thanks to it. Metal/chrome bumpers, ironically, made removal easier and if not, it didn't hurt much and you just cleaned it up.
Modern designs just suck. (Old car nut here, too.)
I put the "bumper" stickers on the body.
My last car (a Ford Aspire) only had a fairly narrow strip of metal on the back, what with windows, tail-lights, lift handle, license plate depression, etc.
My current car (a PT Cruiser) has all the room in the world for magnetic stickers.