WebUrbanist blog has a neat round-up of their series of 8 posts about guerilla marketing: its history, evolution and acceptance (or exploitation) by major corporations - this is the article for you.
Here's something interesting that I've just learned, the difference between guerilla marketing and viral marketing:
There’s a subtle difference between viral marketing and guerrilla marketing. Guerrilla marketing ambushes the viewer and grabs their attention when they least expect it. Viral marketing is advertising that you voluntarily pass around because it’s cool, not necessarily because you want to help build publicity. There is some noticeable overlap where a guerrilla ad that doesn’t quite look like an ad is so great that you to tell other people about it. But viral campaigns can’t be engineered in the way that other types of advertising can - they depend solely on the user to deem them worthy of passing them along.
It doesn’t take a marketing genius to figure out that people accept suggestions from their friends better than from some nameless marketer. It’s also apparent that campaigns which engage usually enjoy more success than campaigns that educate. So logic would follow that engaging, ground-level marketing campaigns are one of the best ways to present an idea.
Link - Thanks Kurt!