Matthew over at the code|hack|create site needed to be aware of movement in many places at once, in order to detect the arrival of the courier and movement in other areas of his building. Although placing passive infrared sensors around and wiring them back to a main unit sounds simple, in reality all that wiring is inconvenient and a mess. Therefore Matthew instead connected each PIR to an Arduino and an inexpensive wireless data transmitter. At the receiving end another Arduino to a matched receiver which can then take action once motion in any of the areas has been detected.
If you're interested in using wireless data links with your Arduino project, we have two convenient receiver shields - both 315 MHz and 433 Mhz. Combined with the Getting Started guide you'll be up and going in no time. For more information check out the product page.