I love the idea of integrating information feedback into one's environment. Ambient Devices has done some interesting work with this concept, but their devices still aren't invisible. But putting a display mechanism into one's contact lenses certainly could be! A project at the Center for Fluorescence Spectroscopy is designing "a contact lens that would change colour like a traffic light - from green to yellow to orange to red - enabling the wearer or an observer to determine a [broad] range of blood sugar level, from too low to too high." It's a wonderful idea, and apparently not as far fetched as it may sound.
(from Contact lenses that react to blood-sugar levels and Contact lenses react to blood-sugar levels.)