Sekai Camera is the killer iPhone application, just point your camera anywhere around you to have instant feedback about whatever is on your screen. Directions, reviews, messages, offers, and any tag you can imagine will appear floating in front of you, adding a graphic overlay to the world. Using the iPhone 3G, camera and the GPS to the internet connectivity and its microphone. you can create social tags on the world. see information about places and objects around you, overlaid on the real-time video. and add your own information.
Take a look at Tonchidot – tagging the world