Arnold Kling linked to an informative and entertaining lecture by Bill Bishop on political/cultural clustering in America. Bishop showed that neighborhoods are more polarized today than they were in the 1970s; there has been an increase in the percentage of neighborhoods where the difference in votes for the Republican or Democratic presidential candidate was greater than 20%. But of course people aren't checking the vote records of the neighborhoods before they move in. People are instead attracted to a similar culture. Surveys on questions such as the morality of physically disciplining children and on non-married cohabitation rates are great predictors of voting.
I was in San Francisco recently and the political polarization was obvious. There were Obama volunteers everywhere, selling T-shirts and badges and exhorting people to vote. I saw them every day and I never saw any pro-McCain advocacy at all (though I did spot a few old Ron Paul stickers).
He concluded that this is bad for national unity. Come to think of it, I had a very interesting conversation with a guy writing a book advocating the secession of California, so yeah, it's bad for national unity. But national unity is overrated. I prefer national discord.
On the plus side, cultural diversity is a good thing, right? People are always complaining about the homogenization of the world into an empty monoculture by America's cultural imperialism.