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.

Click this link to see if your i5 is part of the recall (mine is).  If you require a loaner during the time that your iPhone is being serviced, participating Apple Retail Stores and Apple Authorized Service Providers will have iPhone 5 models (16GB only) available.  Note that this option is not available for mail-in repairs.