many different version of raspberry pi out there, so what have you got?
cat /proc/cpuinfo
You will see your device data including:
Hardware : BCM2708
Revision : 0003
See here to find out what you have … http://elinux.org/RPi_HardwareHistory
Why is this important? – Well GPIO pins are numbered differently between versions … http://www.raspberrypi-spy.co.uk/2012/06/simple-guide-to-the-rpi-gpio-header-and-pins/