Apple is doing a replacement program...
https://ssl.apple.com/support/iphone5-sleepwakebutton/
The site linked above has complete details on checking if your phone is among those eligible for the free repair, and how to go about getting it.
Checked... Mine is among those eligible for a repair... I've got no issues with mine... yet...
https://ssl.apple.com/support/iphone5-sleepwakebutton/
Apple has determined that the sleep/wake button mechanism on a small percentage of iPhone 5 models may stop working or work intermittently. iPhone 5 models manufactured through March 2013 may be affected by this issue.
Apple will replace the sleep/wake button mechanism, free of charge, on iPhone 5 models that exhibit this issue and have a qualifying serial number.
The site linked above has complete details on checking if your phone is among those eligible for the free repair, and how to go about getting it.
Checked... Mine is among those eligible for a repair... I've got no issues with mine... yet...