Geek Graffiti
It's amazing how much interesting and funny stuff can go by on the 'net in the course of a few years, and amazing how much of it any one person is destined to miss.
Last night I surfed and read a few things about geek graffiti ... urban nerds making public statements with LED's and magnets, graffiti circuits painted directly on walls with conductive paint, giant projectors putting interactive animations on buildings, etc.
This one hack of postal envelopes is certainly illegal and a waste of public resources ... but funny and fascinating not least because of its subtlety. You have to look pretty close to even see that the perpetrator did anything.
Fun links to see things about geek graffiti: