As Brian mentioned, Joe Romm has laid out a blueprint for how the world could theoretically hold atmospheric carbon concentrations in check with existing or soon-to-be-existing technologes. Doing all that seems incredibly difficult, although as a number of studies have shown, the actual monetary cost involved would likely be quite small. One note, though: Romm's analysis doesn't seem to mention changes in land-use patterns as a means of curtailing emissions and energy usage. (If he factored it in and I missed it, apologies.) This seems like a pretty significant part of the story.
A 2003 World Bank study comparing various cities in the United States illustrated the dramatic difference a bit of sprawl can make. Boston, for instance, isn't the most compact city around, but if its population was as spread out as, say, Atlanta's, then Bostonians would be driving about 9 percent more, kicking up a lot more carbon into the air. If Boston had Atlanta's inferior rail system, driving would increase another 5 percent. In fact, if you could somehow wave a magic wand and move the entire population of Boston to a city with all of Atlanta's sprawl-like characteristics, total driving would increase 25 percent.
Now, some amount of sprawl might always be inevitable, since many people don't enjoy living in crowded urban areas and may well want low-density subdivisions and industrial parks and freeways. That's fine. But that doesn't mean it's impossible for urban planners to constrain sprawl. Compare Vancouver and Seattle. Similar cities in similar areas with similar sorts of people. Yet the former has promoted downtown development and limited freeway expansion and, as a result, has considerably less sprawl. As that World Bank study suggests, that can really have a dramatic effect on emissions.
These patterns also seem to adjust pretty quickly to carbon prices. NPR recently reported that, as gas prices climbs, the pace of sprawl in northern Virginia has taken a breather, as people are deciding they don't want to live so far away from urban centers: Home prices in the outer suburbs and exurbs have been falling much faster than the regional average, while home prices closer to Washington have actually been rising. (Some people seem to be thinking that maybe it's not cheaper to buy a lower-priced house way out in the suburbs if you end up spending so much extra time and money on transportation.) There are things local governments can do to accelerate this trend—increasing the availability of affordable housing closer to the center, for starters. I'm not sure how much, exactly, all this would contribute to a larger emissions-reduction strategy (a half-wedge? a quarter-wedge?), but surely it'll play some part.