Apple repairing iPhone 5 sleep buttons for free, even if you're out of warranty.

You need to go to this webpage to check if you're eligible:

https://ssl.apple.com/support/iphone5-sleepwakebutton/

I'm guessing it was a specific batch, or period of production which caused the sleep buttons to fail.

It will take a few days to fix, but as a bonus they're giving out loaner iphone 5's for that time.