Hi All I got the same issue a day ago. Took it to the Nokia Care service centre. They had a look and said the key is to be replaced and will cost INR 650 + tax (approx USD11)!!! I took the phone back without servicing. Came home and opened the thing apart and had a good look on the switch. The culprit actually is no key or switch, but the solder that holds the switch to the board. There is so little solder on the terminal which is supporting the switch against the countless lock/unlock presses!!! Eventually, the solder comes off and makes the circuit open making the switch non functional. I just applied little more solder and made the dry solder joint good. Now it works like a charm! :)