I've had the same problem on both sides of my S3, the passenger side one I could repair, but the drivers side one needed a new one, so I paid the £75 for a replacement, don't forget to buy 4 new screws also for 27p each.
For those who want to know why the strips comes away from the door, basically, the actual skirt/door moulding is made of plastic, and theres like a strip made of some type of nylon plastic with 4 brass bolts in them where the screws go into. This strip is held in place onto the moulding by a rubber seal, and this perishes away, or shrinks with age, letting the nylon strip part work it's way loose.