From a lengthy US News article:
With corporate evangelism, the goal is to find and identify those customers who are already crazy about your product or service–who are actively talking it up in blogs or Web forums, for instance–and turning them with loads of personal attention into "customer evangelists" who then spread the word to others, who then–well, you get the idea. A more secular term for these superfans is "influentials," the people the rest of us seek out and trust for advice about what cars, computers, and clothing to buy. So evangelism is a way of actively creating word-of-mouth advertising or marketing, turning your passionate, influential customers into a volunteer sales force.
It’s great to see a mainstream publication like US News paying so much attention (at least via this article) to the concepts of customer evangelism!