I don’t think this is new since it sounds really familiar to me, but I don’t believe I’ve mentioned it before, so here it is now. STAMPs (Link):

This work targets specifically a human activity that we defined as Collaborative Annotations of a Map in a mobile setting. In general, this can be seen as a form of `Spatialised’ communication, which means that communication makes explicit usage of the geographical/physical context as referent to the message content. The goal of this study is to develop modeling schemas that enable to integrate spatial information, as embedded in maps, with the textual information produced through computer-mediated communication.

In English:

STAMPS is a little program. It can run on your Mobile phone. Using this program you can see a map of the place where you are, visualised on the screen of your mobile. There, you can write a kind of SMS and attach it to the map so that other friends can see your message appearing on their map. You can write for instance: “this is my preferite pizzeria!”, to offer advice to your buddies. All the messages left in the system say something about the city where you live: what are the sport locations, the place to eat, the meeting spots. After a while, we want to use all these information to help the users to navigate the city. You can ask the system, for instance: “where is a pizzeria near by?”, and the system will search for other people’s messages which refer to the term pizzeria to give you an advice.

Just wait til someone embeds hardware into cell phones so that people can add smell. When that happens, just remember: on the cell phone network, you never know who’s a dog.