There is a very interesting debate going on in Steven Cohen's Library Stuff about the role of blogs as an alternative medium for academic publishing. Although Steven Bell has makes some reasonable points, I tend to agree more with Steven Cohen. Speaking for myself, it's a fallacy to think that if I transferred all the time & effort that goes into my blog into more traditional writing - that a couple of publishable articles would come from it. Writing an article/paper for publication is a such a big & intimidating undertaking that it deters most ideas from ever reaching any audience.
I think that the differences in the two media tend to make web logs postings and journal articles very different from each other. This isn't an absolute, just a tendency.
So are web logs and journal articles different but equal? Well, in terms of pure prestige - generally not. Because there being no gate keeper and editor, because of the existing permanency and stability problems. Of course there can be exceptions.
But does that make web logs useless? Far from it! In the same way, an encyclopaedia has more prestige than a journal article yet that doesn't make journal articles useless - because journal articles are more current and can cover wider range of subjects. I think that blogs are to journal articles what journal articles are to encyclopaedia.
Blogs are also rougher, generally shorter and less formal than journal articles (although they certainly don't have to be). The thing is that although there is a LOT of rubbish in web logs (including this one, see my Matrixy post), there are also some pearls which cannot be found anywhere else. Another advantage blogs have is their spontaneity and vitality, which are good things. Bloggers are still a very elitist group, but when compared with the select few who are published authors - we are the unwashed masses who deserve a voice.
This is not a zero sum game. Blogs already have a place in the corpus of knowledge, and this place is growing. They're not going to replace journals, encyclopaedia and books, which is also reassuring.