started seeing him on the pavement in chicago last year. at first i didn't want to look into it but after seeing him in san francisco i had to find out something. it took a little digging but i found a fairly large flickr group with some details in the discussions section. there's a link to here with this minor detail: he's been in more than a few major cities. have you seen him?
yes his stuff is all over boston. A friend of mine in chicago takes photos of his stuff wherever she goes, and I've tried to make an effort to take some around here. They are also on electrical boxes and some are made of other materials. A few of them on my walk to work are much larger as well...I think they're pretty great.
Then there are the toynbee tiles: http://en.wikipedia.org/wiki/Toynbee_tiles I've seen one or two of these in town. phil
