Apple has finally admitted that there's a slight issue with the reliability of the sleep/wake buttons on a small percentage of iPhone 5 handsets (specifically those manufactured after March 2013) and will offer users replacements free of charge if they're affected.
If your handset has been acting up when you're attempting to bring it out of sleep mode, then you need to head on over to Apple's dedicated support mini-site to find out if you're eligible for a fix. All you need to do is go here and enter your iPhone 5's serial number to see if you're covered. If you are, and your sleep/wake button has been acting up, Apple will replace it free of charge, giving users the option of dropping into their local licensed Apple Store/retailer (you'll be entitled to a loaner as a temporary replacement while your handset is being repaired) or by mail.
The relacement scheme opened in the USA and Canada on April 25th, and goes live in all other countries on May 2nd.
Those who have paid to have this issue fixed may be entitled to a full refund by Apple, and are encouraged to get in touch with the company to discuss any potential reimbursement.