I developed this concept as "Digital Graffiti" several years ago. A range of equipment would broadcast info via Bluetooth, infra-read, RFID, Wifi as snippets of XML. The idea was to develop a device that was cheap enough that after you loaded it with text and multimedia, you'd be free to attach it anywhere in a public space. I figured that a device with 32M of storage at a price point of $1.00 USD would be the tipping point (customers wouldn't mind creating a message and 'throwing' it away at this price). Of course, this could become extremely obnoxious as solar-powered devices might squawk for years in public venues. I assumed that some kind of filtering mechanism would be developed to select the signal from the noise.