Siemens has developed a neat little app - an invisible digital graffiti service, which creates site-located electronic text post-it notes using almost any mobile device (with a GPS add-on). Leave a virtual note for your friends: "Waited for thirty minutes, left without you. Loser." Although the technology is about two years from the market, I think I can guess two prime groups of early adopters: trainspotters (Guardian, r.r.) and geo-caching afficionados.