Oops... I actually copied and pasted that but meant to have more text before the paste. That's a real ad, and I wanted to use it as an example of the DANGERS of guerilla activism.
It can easily become guerilla marketing--at first for good causes! Then maybe to add a little money to the coffers, but ONLY for good causes like Greenpeace. Then just a LITTLE more of it for companies who aren't so great, but ONLY to get more money to protest against something nearly as bad as the company is ... and then pretty soon you're in the advertising industry. Except you get paid far less than you deserve, and you're actually causing MORE eye-pollution, because even the most evil ad agencies have to keep their ads limited to billboards and television commercials. Pretty soon, you're Shepard Fairey.