Far from encouraging mobile phone users to deface public property, "digital graffito," as Siemens had dubbed the technology, would allow people to affix virtual information to locations. Tourists would be the most obvious benefactors of the new technology that could send historical information to their mobile phones, notebooks or PDAs as they make their way past city landmarks.

Users can access a city map marked with nearby locations where they can pick up the digital messages -- text, sound or images. And when they're on or near the spot, they can receive the digital graffiti directly on their displays.

The difference between the new technology and text messaging is that the digital graffiti isn't sent to a particular address. Instead it can be received by one or more people when they reach the physical location defined by the person who sent the message. To set a message, one merely sends it to a server, where it is saved and affixed to the location.

Invisible guides

Siemens developers have shown it works with a prototype completed at the University of Linz and the Ars Electronica Center in Austria.

"In two or three years digital graffito will be ready for the market," according to project head Gerd Kölb said.

But will users be ready? Digital graffito can only locate messages with mobile devices outfitted with Global Positioning System (GPS), which isn't quite the industry standard. And it will still be more than a few years before all mobile phones have GPS, Andreas Tasch of Munich's Technical University told the Süddeutsche Zeitung daily.

In a year-long trial involving 100 people, the Munich paper tested the technology, and although the testers were able to locate each other, they weren't able to leave their own virtual graffito for others to receive. They also criticized the system for frequently being slow.

"The testers all found the principle of the technology great, but the technology just isn't polished," Tasch said.

The Süddeutsche Zeitung identified other potential problems that will need resolving: In order to show people where to find the invisible messages, the server must retrace their position every 30 seconds, a nightmare in terms of data protection. And companies could easily engage in nasty marketing battles, leaving disparaging virtual comments about the competition throughout town.