And what about sticking them on a fridge, filing cabinet, or other metal surface? If my wife's phone had a magnetic backing, she could slap it on the door to our garage, thereby increasing her chances of remembering to take it to work. (Don't tell her I said that; she'll kill me in my sleep.)
To paraphrase the ol' saying, there's a Kickstarter project for that. MagSkin adds a magnetic backing to your iPhone 4/4S, thereby giving it an adheres-to-metal personality. (What, you thought I was going to say "magnetic"?)
According to Weyer, the skin is less than a millimeter thick, but strong enough to "fully support the weight of the smartphone when placed vertically against a refrigerator or other ferrous metal."
Assuming the project gets fully funded, Weyer expects MagSkin to ship in September. Most of the early-bird options are sold out, but as of this writing, you can pledge $15 or more to get a MagSkin in your choice of colors, shipping included. (Weyer hasn't decided on the final pricing yet, but says it will "probably" be $19.95 -- including two "docking magnets" to allow the MagSkin to be used with non-metallic surfaces.)
You could, of course, buy inexpensive magnetic sheets and cut your own MagSkin-style backing -- but it wouldn't look anywhere near as cool as Weyer's.
Personally, I rarely find myself wishing for a way to adhere my iPhone to a metallic surface, but I can certainly see the appeal for some folks. If you're one of them, well, consider funding this project!