Have a look at the following links.
http://www.raspberrypi-spy.co.uk/2013/01/cheap-pir-sensors-and-the-raspberry-pi-part-1/
I have also created a Arduino 2 Raspberry Pi Bridge shield that lets the two devices communicate through Serial.
http://blog.protoneer.co.nz/arduino-to-raspberry-pi-bridge-shield-pcbs-finally-arrived/
Hope this helps.
Regards,
Bertus,
REGARDS
PYT