I finally got around to writing up my first Arduino project.
http://fischco.squarespace.com/blog/2009/3/17/arduino-rangefinder.html
Its an ultrasonic rangefinder that is positioned above my desk at work. I've got some buttons to adjust the detection range. When it detects a state change, ie "Here" or "Away", it sends a signal to a simple Processing app, which in turn hits a web service and logs me as at my desk, or away from it. Then people can visit a web page to see if I'm there or not!
Not terribly useful, but it was fun. 