Apple have responded to the complaints from a few people that the sleep/wake button on some iPhone 5 models have a problem and have set up an exchange program.
The link below takes you to their page. US and Canada are already live with other countries following on May 2nd.
Basically you need to take the offending machine in to an Apple Warranty Repairer or post it to them and it will be replaced. Backup into iTunes first guys!