There are many unbelievable things about Captain America's character, but if you can believe there's a metal called Vibranium and a Super Soldier Serum then the electromagnetic action on his shield should be no big deal.
And as you can see in this 60 Seconds Of Science video Captain America's shield and electromagnetic bracer make up one of the most realistic weapon systems in the Marvel Universe.
Ontario Science Centre educator Russell Zeid and engineer James "The Hacksmith" Hobson discuss the real science behind Captain America's electromagnetic shield, and demonstrate how it would look in real life. Only two words to describe it- freakin' cool!
-Via Laughing Squid
One caveat is that at distances larger than the size of a magnet, the force will start to drop off as 1/r^4, which is really fast. If you had an unobtainium small magnet that can pick up a car at 10 cm, it would struggle to pick up a paperclip at 3 meters.