I don't use the power button on my Macbook Air as I have hot corners setup to sleep or I can use Alfred to sleep or shut it down.
With PowerKey installed you can remap your power button to perform any of the following actions:
- No Action
- Delete (backspace)
- Page Up
- Page Down
If you are running Mavericks (10.9.2) and looking to remap the power button then you will have to run a Terminal command first to remove the immediate sleep action that it currently performs.
Add the following to Terminal and hit Return.
defaults write com.apple.loginwindow PowerButtonSleepsSystem -bool NO
Once you have entered this logout, login again and then open the PowerKey app to change the power key function.
If you don't want to remap your power button but want the button to behave the way it did before the 10.9.2 update then the command above will do that for you.