Well, that didn't take long. It's been less than a week, and I'm already feeling quite familiar with the Arduino platform. I just mashed this circuit together in less than 5 minutes (in fact it took longer to video tape than it did to make)!
I picked up the new Parallax motion sensor circuit yesterday for $10 from Radio Shack. Parallax has done a great job of making these chips accessible and easy to use by making the documentation available online. I would recommend all parts vendors to be this accessible, in fact, since it makes circuitry approachable to the novice, and reduces headaches for experts too:
Here's a video of the motion sensor "in action" (pardon the pun):
The device only has 3 pins: one that connects to ground, one that connects to +5V, and a signl pin that I've connected to Arduino pin 8. Then, I've connected an LED to Arduino pin 5, which I turn on for a second whenever the motion sensor is triggered. Here's the source code.