From what I've read, bees will travel 2.5 to 3 miles for food, so in a city setting I would think the bigger issue is neighbors and city ordinances. If you have a large enough lot to place and position the hive(s) in such a way as to not be a problem to neighbors, etc then its a grand plan.
Cities, in my mind, are one of the best places to place hives. There is a lack of the mass pesticide use that one finds in the rural and agrarian areas of the country, fewer predators (skunks/bears) and for those living in the city, it is more convenient. On top of that, the majority of cities have a plethora of bee-friendly flowers -- unlike many rural areas covered in mono crops. A balanced, healthy diet, safety from evil-doers and convenience for the keeper make city beekeeping a wonderful idea.
Good points. Bees will reach saturation levels much more quickly in urban areas too though. Be very careful to check first on local ordinances...thre are fines associated with violating them, and sometimes even jailtime associated. Bee sure your t's are crossed and your i's are dotted first, and DON'T assume that you don't need a fence to keep bears out (personal experience).