Passive IR motion sensors detect motion of warm objects -- like humans or other mammals -- using infra-red light.
A PIR sensor works like a digital input -- like a switch. You can wire its middle pin to any Arduino digital I/O pin, and wire the other two pins to 5v and Gnd.
Then read it just like your push-button.
Credits: This page uses a circuit image cribbed from LadyAda's detailed PIR motion sensor tutorial, and that's where I bought the IR motion sensors.
There's a lot more to know about PIR sensors, and you can make your sensor a lot smarter so it only triggers for real events. Read the LadyAda tutorial to get the details!