Looking for trouble
Every weekend there's a line out front of the Avis rental car window on the upper west side of NYC. Every weekend, ostensibly computer-literate upper-middle-class yuppies waste hours trying to pick up a car when they could just use Zipcar.
For this person, in this moment, a message about zipcar is not only not spam, it's a gift.
Worth wondering why the company doesn't have someone standing out front with fliers.
Along the same lines, why doesn't the local accountant sponsor the business section of the nearby independent bookstore? Slip a bookmark and business card into every personal finance book the store sells... it's the right message at the right time.
Blogs, of course, ought to be the perfect place to find people in trouble. The challenge is in getting past the "I won't click on an ad" mindset that 80% of those online carry around. Guerrilla marketing works best when it takes the form of a sponsorship or other unexpected combination of advertiser and content. Blogs let you go farther than that, though.
The most effective marketing use of blogs seems to be when the advertiser/marketer uses the blog as an opportunity not to sell a product, but to attract people who are in the right mindset. Joel Spolsky rarely writes about his product, but that's fine. The people who read his writing are the very same people who need his product, and his proximity to the valuable ideas (and his reputation) makes it not such a leap to go ahead and buy what he has to sell.
Attract people in trouble-->Help solve their problems-->Build your reputation-->Sales happen.