This question has been addressed by many people already, and I really don't have anything new to add to the topic, but since an article I wrote over a year ago has finally hit hard copy I thought I'd share it with you anyway. I was pleased, when looking at it again a year later, that the reasoning still stands up. A couple of the URLs mentioned in the article are gone, but hey, that's the web for you. Also note that Haworth doesn't allow the use of contractions in their journals, so the writing is a little more formal than I would've preferred. I'll be updating this post with the standard disclaimer that Haworth says I need to include, but here's the article:
Pival, P. (2005) Blogging for the Distance Librarian. Journal of Library and Information Services in Distance Learning, 2(2), 55-63.
Interestingly, today brought yet another story about how bad it is for Academics to blog - this one from Slate, Attack of the Career-Killing Blogs. I guess because I tend not to post many personal or political thoughts I don't feel worried about my blogging activity, though I too am up for tenure next year. Maybe I'll be able to post that the committee found my blog to be a favourable factor in their decision :-)