We first tried to remedy the situation with instant glue but that did not help. We cut a piece from a toothpick and squeezed it in to support the broken tooth. After tweaking a little bit, we finally solved the problem.
A message to Samsung engineers:
1. Why don't you make load bearing parts like the power switch from strong materials instead of lousy plastics?
2. If the N150 default was to wake up when the lid is opened (like our other laptop, the Samsung RV511), the slider wouldn't be used that much and it would last much longer.
Update (8 June, 2012): As said in the comments, you can set the lid open behavior to wake up in BIOS. Restart your netbook, press F2 to enter BIOS, go to advanced tab and change Lid Open S3 Wake-up from Disabled to Enabled: