Have any of you seen tireflys? A guy in my office building has them. I only ever see his bike locked up, so I thought they were some sort of tire-pressure monitor: they are sci-fi looking, bulbous blue doohickies that screw onto the valve of the tire tube -- but I asked him about it and he showed me how they work. They are motion sensitive, so when you start riding, they light up. Here's an image from the manufacturer. I don't have them, but I am very tempted.