Lately there has been a fair bit of talk (here, for example, or here) about whether blogs, particularly those written by scientists, are actually changing the scientific discourse and the way science gets done (particularly in terms of debating controversies or resolving disagreements). I was recently asked this by a science journalist, too.
My short answer is, "maybe, sometimes, but mostly 'no'." (Thus, I am roughly consistent with the old adage that article titles posed in the form of a question are almost always answered by "no".) The main reason that blogging is, in my view, not having some major transformative effect on science is that the vast majority of scientists do not blog, and a slightly smaller (but still vast) majority do not even read blogs let alone comment on them or ponder writing one. Blogs are still far and away the exception rather than the rule in terms of how scientific discussions take place.
That being said, when the relevant participants participate in blog discussions (or the equivalent, as on mathoverflow), very cool things can take place. However, I think the most productive version of this happens either when someone really tries to educate an interested audience (my attempted model here most of the time - a sort of science journalism by scientists), or when informed discussion happens between knowledgeable experts (sort of a virtual version of the kinds of conversations that can happen at good conferences). I do think that unilateral discussions of controversies can serve a useful purpose. However, one-sided presentations on the internet are not all peaches and cream, as you no doubt know.
(A mildly amusing note: My previous post got a big spike in pageviews thanks to Physics Today tweeting the link. Thanks, PT! Hopefully some of those people will stick around. Of course, my most-viewed post of all time, by about a factor of 3, is still my commentary about whiskey stones. Clearly I should routinely stake out an aggressive position on some physics point connected to good Scotch.)