Fun concept, but I immediately imagine someone going around smashing these with a hammer (or, perhaps more likely, city workers being sent out to remove them). :\
Here's a much more expensive project with a similar idea:
Bluetooth low energy is option once more phones support it. With a button cell the transmitter will operate more than a year. Hide it completely in a wall or somewhere else where it is difficult to remove.
Here's a much more expensive project with a similar idea:
http://torrentfreak.com/kiosk-of-piracy-an-offline-copy-of-t...