Hi all,
To improve the efficiency and minimize load to the CPU, is recommended to use interrupt method instead of While loop method in the Shutdown script.
The code for the interrupt method is this one:
#####################################################################################################
# Import the modules to send commands to the system and access GPIO pins
from subprocess import call
import RPi.GPIO as gpio

# Define a function to keep script running
def loop():
raw_input()

# Define a function to run when an interrupt is called
def shutdown(pin):
call('halt', shell=False)

gpio.setmode(gpio.BOARD) # Set pin numbering to board numbering
gpio.setup(7, gpio.IN) # Set up pin 7 as an input
gpio.add_event_detect(7, gpio.RISING, callback=shutdown, bouncetime=200) # Set up an interrupt to look for button presses

loop() # Run the loop function to keep script running

#####################################################################################################

Here it uses the PIN 7, if you use another pin (Like 17) just change the PIN number in the Script.

I found this code from tutorial to add an On/Off switch to my Raspberry and it's works great.
With thid method, I added 3 Buttons to the Raspberry:
1 - Soft Off
1 - On
1 - Hard Off
With this 3 buttons. the power supply is more likely a ATX power Supply.

If you are insterested, contact me with personal message.

* Extracted from raspberry-pi-geek.com