The urbanist proposal isn't "hey, jerks, why don't you all move to dense downtowns." Rather, the proposal is something like "why don't we impose carbon taxes so that things like driving long distances and heating or cooling large detached structures are priced in accordance with their social cost? Why don't we stop having the federal government heavily subsidize driving cars as the preferred mode of transportation? Why don't we have more areas that allow for high-density zoning, thus reducing the cost of urban housing?" It's not that we urbanists are unaware that many people live in low density areas because its cheaper, it's precisely that we are aware of this fact that makes us believe that the "traditional unipolar downtown" could make a comeback.
I'm all for making McMansion owners and SUV drivers feel more of a pinch, if we can find a way to do it that doesn't penalize working-class drivers and homeowners, and I would definitely support having more urban areas that allow for high-density zoning, so that middle and low-income workers whose jobs require them to be downtown from 9 to 5 don't have to commute long distances if they don't want to. (Of course, I'm against an awful lot of zoning regulations, which is a place where I tend to part ways with the crunchy-cons.) But I don't think we should make "rebalancing in the direction of urbanism," as Matt puts it later in his post, a major policy goal; I think suburbia is a great (maybe the great) American socioeconomic achievement, whose virtues far outweigh its vices, and that using the levers of government to encourage families to leave the suburbs would represent a deep betrayal of what I take to be the heart of the American Dream. (Which is a cliche, sure, but also a reality.) When it comes to global warming, therefore, I'm all for telecommuting and fuel-efficient cars and various other ways to reduce our carbon footprint; I'm not for any plan that stands athwart suburbanization, yelling stop.