First: Will rising gas prices inevitably push people into the cities as they become desperate to cut down their commutes? Maybe, but it's worth keeping in mind that commutes go in both directions these days. There are plenty of jobs in the exurbs (Joel Garreau's "edge cities"), and although individual circumstances vary widely, this means that an awful lot of commutes today are entirely voluntary. As gas prices go up, workers will start taking jobs closer to home (wherever that may be) or will move to be closer to work (wherever that may be), and commuting will be reduced substantially without any change in infrastructure or land use planning at all.
Second: A focus on increased density is going to mean a funny political switcheroo for a lot of liberals. We're mostly accustomed to fighting evil corporations on behalf of the little guy, but it turns out that most suburban (and many urban) zoning regulations have been put in place by exactly the little guys we're used to teaming up with. Developers, on the other hand, would happily build out every last acre to the maximum possible density and maximum possible profit if only they were allowed to. So if we're in favor of higher density, we're mostly going to find ourselves siding with big developers and very much against local public opinion — and believe me, you haven't really taken on the task of changing public opinion until you've sat through a planning commission meeting trying to out-talk an angry mob of homeowners who are dead set against a proposed zoning change that might affect their property values by 1%. Strange bedfellows indeed, but those are the bedfellows we're going to have to get used to.
Third: What will higher gas prices really do to the burbs? Well, let's suppose that over the next couple of decades the price of oil (adjusted for inflation) goes up to $400 a barrel. No — screw that. Let's say $800, with gas at 25 bucks a gallon. Are suburbs doomed?
Well, aside from prompting development of lots of alternative energy sources, sky-high prices like that will cause us to use less gasoline. At a very conservative estimate, prices like that are likely to reduce driving by 40% (about to the level of suburb-friendly Australia right now) and increase average fuel efficiency by 40% (about to the level of Europe right now). Taken together, this means we'd use about a third as much gasoline per person as we do now.
Today, the average American spends about $2,000 per year on gasoline. So, if the price of gas goes up to $25, but consumption of gasoline goes down by two-thirds, that means the average person will be spending about $4,000 per year on gasoline. That's a difference of $2,000 — not pocket change by any means, but certainly something that most suburbs can can live through. They may be suburbs with more light rail and better bus service — as well as more apartment blocks and taller office buildings — but they'll still fundamentally be suburbs.
Anyway, that's it. Like I said, just some random thoughts. And pretty much in tune with sensible stuff like this. As Robert Bruegmann has pointed out, suburbs aren't just some American invention of thehigh-flying 50s: they've been around for a very long time, in pretty much every region of the world. Cities and suburbs will both see changes over the next few decades, but my guess is that those changes will be less dramatic than a lot of people think.