As of April 25, 2014, the replacement process will be available in the U.S. and Canada. In other countries, it will be available as of May 2, 2014. Replacement of the sleep/wake button mechanism is done at an Apple Repair Center. There are two ways to send your iPhone to the repair center—carry-in or mail-in. See below for details. The replacement process takes approximately 4-6 days from the time your iPhone is received at the repair center until it is returned to you. Your iPhone will be examined prior to any service to verify that it is eligible for this program and is otherwise in working order. Before sending your iPhone to Apple for service, you will need to back up all your data and erase all your content and settings. Note: If your iPhone 5 has any damage such as a cracked screen which impairs the replacement of the sleep/wake button, that issue will need to be resolved prior to the sleep/wake button replacement. In some cases, there may be a cost associated with the repair.
Must have been a common fault then. I've had a sleep button die on my old iPhone 5. Luckily my phone was still under warranty so I got it replaced at an Apple store. Also happened to two other people I know.