Infrastructure is another problem. Public transit, in particular, faces a chicken-and-egg problem: it’s hard to justify transit systems unless there’s sufficient population density, yet it’s hard to persuade people to live in denser neighborhoods unless they come with the advantage of transit access.
This is all true enough (and what Duncan said) but before we try to run in terms of transit and infrastructure it makes sense to walk. Many Americans live in places where there is no transit infrastructure, and a healthy number of people live in places that just aren't well-suited to creating any such infrastructure. But it's a big country and some people already are living near transit infrastructure.
One piece of very low-hanging fruit is to promote denser development near our existing stations (instead of, e.g., bungalows and vacant lots near the Brookland Metro here in DC) which are often places where developers are clamoring to build and potential residents clamoring to live but incumbent property owners are trying to avoid sharing the wealth. Another opportunity is to improve service quality and frequency at things like our existing commuter rail lines. With gas prices rising, I bet a lot of Virginians are giving VRE a second look but they're probably discovering that it sucks. Making an existing commuter rail line more useful isn't brain surgery and doesn't involve any paradoxes or dilemmas, it just needs to be made a funding priority.
Last, it's always worth reiterating that while a lot of Americans live in genuinely low-density environments, many car-heavy parts of the U.S. are actually pretty dense. New Jersey is, as I've noted before, about as dense as the Netherlands which is one of the least driving-oriented countries. Los Angeles, too, though far from the densest city in the world is actually pretty dense and once featured a lot of transit in the form of streetcars while the well-designed Portland is hardly the second coming of Tokyo. If we start doing better with the relatively easy cases, that would create a more supportive environment for more difficult issues.
Photo of New Jersey Transit station by Flickr user Morrissey used under a Creative Commons license