From the description on their internet site:
To demonstrate the concept of waypoint sharing we have been developing a number of waypoint sharing applications for the project we call Geograffiti. Using the GPSter database, these applications access the waypoint lists for retrieval and storage of waypoint data and other accessory information, such as text, images, audio, video, or links to other information. Running on location-aware and wireless-enabled portable devices, the applications can retreive waypoint lists based on context (search keywords), or location, or both. Typical use would include showing all waypoints in a wide geographic region, as a mapping overlay, or showing waypoints in a specific location that the user is presently in. The latter is the Geographiti concept: leaving location-dependent messages or media to be accessed by others not by a URL, but by the real location itself.
According to my point of view, this project is the closest to what I have in mind. The only difference I can see is that my focus concentrated in grasping urban structures rather that just annotating the world. In this sense more emphasis should be devoted to finding the cognitive tasks that are under the understanding of space and the construction of the mental map rather that on the annotation option.