There is a interesting interview with David Owen who has written, "Green Metropolis: Why living smaller, living closer, and driving less are the keys to sustainability" argues that city life - density - is the key to living greener. His argument says that if you put the people closer together, you can lower the carbon footprint.
It's a compelling argument. Most of our personal carbon footprint deals with our living spaces and driving places. If we lived near where we worked and lived in more compact spaces, we would have density. I do find fault in the argument that density will solve everything though. The interview does not address the main issue with cities which is the gathering of goods and the disposal of waste, not to mention our energy consumption. So although the people aren't traveling as much, if there sushi is being flown in from Japan for their eat-out supper, I have a feeling my backyard garden salad just smashed the sushi's carbon footprint to bits...
Some of the smallest footprints I know are friends of mine in Wisconsin who produce their own power/heat, food, and work near home. It's a far cry from NYC, but there is more than one way to make our footprints smaller... And as for city life, we can all consume less, reuse more, and drive less.