The Wall Street Journal Online has an article on social computing. Studying social computing isn't a new phenomenon (which they admit); I remember papers coming out about MUDs in the mid-90s and at least one person did a thesis on the subject - Masters, I think.
One bit that jumped out at me was this:

As part of the project, researchers will look at archived versions of Web sites to gauge, for example, whether blogs have contributed to polarized political opinions in recent years. "These digital records leave unprecedented opportunities for social scientists to study human interaction," he said.

While the latter is indisputable, I think it's dangerous to assume that what happens on blogs is necessarily the way humans interact. It's a great way to study *online* human interaction, true, but everybody online knows at least one or two people who are normally reasonable people that turn into raving lunatics when they're behind a keyboard.
Furthermore, I'm not sure that political opinions have become polarized only in recent years. The first example that sprang to my mind was Anne of Green Gables - at one point in the first book, I think, Anne asks somebody if they're a Tory or a Whig. (See, fiction can be a good insight into human interactions too.) Even now in Canada, we have 3 "official" parties (NDP, Liberal, Conservative), and at least one other that seems to be up and coming (Green), but the NDP and Greens are currently fringe. The question is essentially still "Are you a Whig or Tory?" One could even lump them together: Liberal + NDP = Whig, Conservative + Green = Tory. Granted, I've not studied politics much, beyond "what everybody knows," so maybe I'm wrong - maybe there was some period post-Anne and pre-now that could have been considered to be de-polarized, but I really doubt it.
If there was any such time, at least in American politics, I would expect that it was in the mid-90s, post-Soviet collapse and before, say, 2001. I would also consider that it was more likely that that period was something like The Phoney War of 1939/40. The polarization was still there, just unacknowledged. Giving blogs credit (or blame) for the apparent recent polarization of (North) American politics is like blaming somebody else for how you're feeling.