Siemens Corporate Technolgy in Munich developed a service to place virtual post-it messages at real locations.

These digital graffiti messages can be received via SMS, when the users walks by a placed virtual post-it. This is a great real world SMS spamming method, allowing the placements of ads where ever you walk around. Of course to park informative SMS infront of artwork in a museum or sights in a city are maybe good uses of the Siemens digital graffiti service.
A prototype of this virtual post-it service was already shown in Linz at the Ars Electronica Center.
More details in Siemens Press-Release (PDF, German).
News Tip: Chris W.