A friend of mine tried together with me to start something similar to what you're describing but for people. We started in the city Bern in Switzerland but added the idea to a wiki hoping someday other people will copy it in other places around the world! Unfortunately it is in German but maybe auto translate is enough?

Here's the wiki: http://youcantalktome.net

And here's some pictures of stickers we printed out and handed out: http://youcantalktome.net/pmwiki.php?n=Main.Vorlagen