Instapundit has remarked on the inability of the media to get stories about blog and blogging right. One of the problems is that they can't tell a blog and a bulletin board apart.
To be fair, the dividing line is a bit grayer than it used to be. Kuro5hin is a discussion forum. It is powered by a codebase called Scoop. DailyKos is a blog. I believe it uses a variant of the same software. Slashdot could be a discussion forum, but it could also be described as a group meta-blog.
The difference between a blog with a large number of comments/commenters and a discussion forum often gets pretty miniscule. There are clear-cut cases of either/or, but there are also gray areas where both apply.
UPDATE: Slashdot is picking up the 58% growth in blog readership story. Some of the comments are interesting and relate to blogging and specifically what defines a blog, others are worthless. As with all Slashdot comments, set the threshold around 3 and rank them by score. You will get the cream without the crap.