Mobotag launched in June 2006!

mobotag: mobile tags

mobotag reveals the hidden layers of a city through an active exchange of location based media and text messages via the cellphone. It's collaborative phone tagging of the city. Part virtual graffiti, part walking tour, mobotag creates a spontaneous and easy way for tagging a neighborhood via the cellphone. Send and view messages, images, videos and sounds. See art, read stories, and watch a hidden layer of the city reveal itself. Respond with your media and participate in the creative expression and mapping of your neighborhood.
By sending a text message to mobotag, with your city location, you begin a interactive tour of a neighborhood. Using a unique geocoding feature, mobotag tells you what other messages exist in your local area.

Technical
Using PHP, Perl, MySQL, and a geocoder, mobotag is able to store information and media files at designated locations. This media can be accessed with ones phone, by typing in a city location. Files from that, or the next nearest location will be sent to your phone. For a step by step break down of a user scenario see this page.