I can also confirm that this is a common issue and will be covered under your apple warranty.
I have had two iPhone 5's replaced by apple for this very reason, they must have used a weak part as a lot of people are having the same issue.
Go to the following page and start your apple service requesthttp://www.apple.com/support/iphone/cont...
The following is where you need to click.
"Apple Support" then "Get Started"
"Service requests or troubleshooting" then "Buttons not working" then select "Sleep/Wake Button"
Within 2 days a package is sent and once you have sent it off a new iPhone will be on its way back to you within 5 days, thats right a new iPhone 5 for a button issue. Must be costing them a lot of money.
Anyway let me know how you get on.