Using an Arduino for motion sensing is a fun project. It's fairly inexpensive, and doesn't require a high skill level. You can trigger an LED, an alarm, or even open a door.
From Blue Smoke Labs:
One of our best selling products is our PIR sensors, they are cheap and relatively easy to set up. But for those new to Arduino it can be a bit of a daunting task so we have some up with this tutorial to cover the basics of setting it up, coding it and in the end you will be making an LED blink when motion is detected.