Most blogs, even today, are essentially online diaries; in fact the proliferation of blogs is directly due to this, since anyone can write a diary, even if not anyone can create web pages. However, more and more, one finds specialized "topic" blogs: political blogs being the vast majority, but also for topics like music, photography, the media, and even academic disciplines like economics, philosophy, and of course computer science.
As I write entries, I am conscious of the fact that my posts are a form of public announcement, and as such should be governed by some of the rules of normal academic public discourse. When I chat with people at conferences about problems and results, I have to be careful in what I say about these conversations, in the same way that I would be careful in one-on-one discussions. However, it seems to me that if I hear about a new result, and the result is not confidential, telling a colleague at lunch is still different to posting it on a blog, mainly because of the illusion that I can control the rate of dissemination of information. Moreover, as with normal conversations, I wouldn't want people to think that everything they told me could end up as a blog entry at some point.
A discussion on these lines has been going on in some of the philosophy blogs. I think this kind of discussion will probably happen more and more as more disciplines turn to blogs as a means for information dissemination and general discussion.