Marketing Sherpa has published a case study on the effect of a blog to the bottom line (btw: the article is only open for public access through September 17) . The company in question is T-Shirt King.
"RESULTS: Site sales tripled during 2003, and have continued strong in 2004. Altogether, blog content helps to bring in about 35% of total site sales -- 10% from the email newsletter, 20% from affiliates who often reuse the content, and 5% from Blog traffic itself.
Blog readers are among the traffic most likely to convert to buyers. "Six to eight percent of Blog readers buy something. Once someone finds a Blog entry and reads the whole thing, they are about as qualified as you can get."