I think for a commercial blog to succeed, it must do the same thing as a successful non-commercial blog, and that means it must be interesting and credible to its audience. In fact I would say a blog is a 'credibility machine'. To use the words of the Cluetrain Manifesto, a blog must speak with the author's authentic voice if it is to be believed... and it is a rare company indeed who can be authentic if all people hear from them is what their marketing and PR department say. Perry de Havilland
This is exciting to read, as I have been musing about credibility and blogging too, and posted some introductory remarks earlier today.