I tinkered a little bit this evening and I have added a motion sensor to the Arduino platform. After that I added a network shield and have coded a small HTTP server... This server provides just a 0 or an 1. 0 if I don't have moved around; 1 if I am dancing on the desk... ;) The sensor works with a radius of about 5 to 7 meters and it is really a lot of fun. You could get such sensors very cheap at ebay, just search for "arduino motion sensor".
I am getting the status every second using wget and are displaying the output in xterm. It looks nice to see how much I am moving when writing this Blog post: