Increases in atmoshperic carbon dioxide (CO2) due to vehicle emmisions are considered one of the most important human-induced factors of climate change. Conventional wisdom would say that urban areas, with their huge populations, dense road networks and congested freeways, are the biggest offenders. This is true to some extent. But, viewed from a different perspective, the per-capita CO2 emmissions for these urban areas can be considerably less than surrounding rural and suburban areas.
Travelmatter.org has posted a series of maps comparing these two conflicting views. Here's a sample from Chicago that demonstrates the sharp dichotomy; both entirely accurate but different ways of analyzing the same data:
In every case, the total CO2 emmissions are much greater in dense urban areas. But, per-capita, the urban areas have much lower emmissions, sometimes dramatically lower. This second view indicates, as WorldChanging points out, that living in denser neighborhoods can reduce your climate impact. It makes sense that living closer to the places you need to go on a daily basis and having more access to public transportation would reduce the emmissions impact. Maybe cities are "greener" than most of us percieve them to be?