I ran across an old report from Forrester that I thought was interesting to read in hindsight: "Blogging: Bubble Or Big Deal? When and How Businesses Should Use Blogs."  There is an updated powerpoint you can download for free (after registration) here.  Here is the summary:

Although Weblogs (blogs) are currently used by only a small number of online consumers, they’ve garnered a great deal of corporate attention because their readers and writers are highly influential. Forrester believes that blogging will grow in importance, and at a minimum, companies should monitor blogs to learn what is being said about their products and services. Companies that plan to create their own public blogs should already feel comfortable having a close, two-way relationship with users. In this document we recommend best practices, including a blogging code of ethics, and metrics that will show the impact of blogs on business goals.

Related reports include:

• DNC Bloggers Missed An Opportunity (2004)
• Marketers: Wake Up to Emotive Network Users (2004)
• Blogging: Not Even Close to Mainstream (2003)
• Emotive Networks Connect Consumers (2002)