Sat 30th April 2005: Over the past few days there has been this guy lying outside the shops on Muswell Hill Broadway and painting the blobs of chewing gum on the pavement. While i was taking the first of these pics on the way home tonight i bumped into a bunch of his friends from the O'Neill's pub across the road from where a batch of these were. They tell me he's planning on going from High Barnet to Highbury or something and that he'll be in the star*ucks down the road tomorrow, so if i can catch him in time i might ask him for a bit of background.
I really should have put something next to this to give you some sense of scale.
I think this one is about 2cm across.
Update later that day: just found this info on Ben here and here.

And here at ponyintheair's blog.

Tues 10th May:
This set just got bOINGbOING'd

Thurs 26th May:
More detail in the local news

Sat 28th May:
Found 16 more.

Wed 13th July:
More refs, if you read Norwegian.

2006... finding more and more.

I've geoTagged a few of these where i can remember the approximate location. See the map link on each page.

Bonus ! :
StefZ has a really nice set over in Barnet