Known defect. If they have the part in stock, it will take about an hour to replace at an Apple Store free of charge. I just got the top replaced on my sister's macbook. The genius says they've improved the quality of the tops, so it should be less likely to happen again. I'm not sure on their policy for out of warranty, though, but even if you are out of warranty, schedule a meeting with a genius and see if they'll help you.