My own sense is that this needs to be handled on a case-by-case basis. Some science bloggers are scientists in their own right (they conduct scholarly research and publish their results in journals–among other places). Some science bloggers are in fact journalists. 

When Andrew Sullivan blogs about science, he’s clearly a journalist paid by The Atlantic. When Tyler Cowen blogs about economics, he’s an academic and holds the position of professor (at George Mason).

That said, as a journal editor, I would have real problems publishing something that had had been already put up on the web in a blog. And as a scientist, I would have real problems with a journalist blogging my results in substance (rather than in summary) before I published them myself.