Reihan Salam had a post the other day where he said "the vocal, influential segments of the center-left are turning against the suburban way of life in quite explicit terms, usually though certainly not exclusively on environmental grounds." Clearly, there's always been a kind of aesthetic critique of suburbia out there (and always will be) just as there are various aesthetic and quasi-aesthetic forms of critique and praise for the countryside and the big city, I think it's wrong to see the rising tide of interest in urbanism as primarily driven by anti-suburban sentiment.
The main direction in which the environmental critique goes, I think, is that there's a large environmental critique of unchecked growth in carbon emissions. Consequently, there's an interest in policies aimed at capping the quantity of carbon emitted in the United States and reducing that cap over time. It's natural enough that this would lead, in turn, to a growth of interests in ways to make reductions of that scale feasible. Reducing the number of families operating on a "one car per person over the age of 15" model and increasing the number of families operating on a "we rent a car when we need one" model. That would involve, yes, a lower proportion of the population living in suburbs.
But it would also -- and probably more importantly -- involve more people living in somewhat different kinds of suburbs. Lots of eminently suburban places feature commuter rail stations or even full-fledged metro/subway outlets. There could be more places like that, and the places that already have it could have more frequent service so it'd be a more appealing option for a larger number of trips. Meanwhile, America has become such an overwhelmingly suburban country that the concept of a "suburb" has ceased to define anything incredibly definite. A suburb can have sidewalks, bike paths, and market-priced parking while parts of many cities are, in practice, difficult to navigate without a car even without huge distances being involved.
On top of all this, we're just past the great urban crisis years of the 1970s and 80s. These days, I worry much less that America's great cities will be rendered unviable than that the urban lifestyle is unaffordable for too many people. If you look at housing prices it's clear that "home in a walkable urban area close to a quality mass transit system" is a product that's in very high demand. Nobody needs to do anything to convince more people to want to live in situations like that. What's needed is for the price of situations like that to be cheap enough for more people to get them. That means some combination of allowing developers to build more units (taller buildings, laxer parking requirements) close to mass transit stations and building more mass transit stations.
The upshot of a situation like that wouldn't be to kill off suburbs at all. Rather, by draining the suburbs of people who actually would prefer to live elsewhere, what'll be left is a situation wherein people who prefer suburbia can more easily afford to live in the very best bits of it rather than in far-flung areas with bad commutes.
Photo by Flickr user OmarOmar used under a Creative Commons license