Francois, I found the pages where people who have found the prints and taken them home after I posted the link and think this is probably much better than leaving them in place. Chances are they would eventuall get damaged and disposed of.
I once thought of bringing some of my prints to the museum and finding a place to hang it without anyone noticing... kinda hard with all the cameras they have
There is an artist who has done just this is various museum.
http://matbenote.com/home/ Not sure how he managed it. Maybe got someone to distract the guards.
Soap
Neat idea. Im seriously thinking about bringing nails and illegally framing my pictures on the outside of peoples houses.
There is an artist (invader) who done something similar and fixed space invader icons made of mosaic tile on public buildings.
http://www.space-invaders.com/Mike