I sorta lost the link I wanted to share, I searched my history relentlessly but to no avail. So I'm going to describe it to you and use some things I did save from the site.
This guy put speech bubbles on advertising around new york. Blank ones, about 15,000 of them. And then people wrote things on them and this guy took pictures.
You can download a pdf of some speech bubbles and print them out, post them around maybe.
WOOOHOO! I found the link. you can tour around his site and see all the photos, some of them are really funny, New Yorkers rock.
I'm surprised I haven't seen any of these around, I mean he printed 15 thousand of them. Judging from some of the ads I'd say he started in early 2002 maybe. I'd put this whole project right up next to dressing the crosswalk guys in my book of cool urban vandalism.