Mar 07, '14 07:00:00AM • Contributed by: MacManager
defaults write com.apple.loginwindow PowerButtonSleepsSystem -bool no
This causes the 'Are you sure you want to shut down your computer now?' dialog to come up much quicker too.
[crarko adds: I tested this, and it works as described. I had to log out and log back in; I'm not sure if that's necessary.]