Back at school now, so we're back to more intermittent posting, now that I'm not spending nine hours a day on the internet. (Extra Bonus Public Service Announcement: Bad idea. Way, way too much internet.)
The weekend in New York has me keen to run an urban campaign, as New York always does. It's organization strikes me as a very reasonable way to write a city for roleplaying purposes. There's a large scale division of the boroughs: Manhattan, Brooklyn, The Bronx, Queens, and Staten. Each of these is a practically a city in its own right, to the point where when I say "New York," what I mean is "Manhattan," because that's where we always go, and that's the part I know. I'm vaguely aware that the other boroughs exist, but I don't have any experience with them, and even a lifelong resident can exist in roughly the same state.
Then there's the mid-scale organization of the named neighborhoods: Soho, Chinatown, Greenwich Village, Midtown, Hell's Kitchen. Each of them has its own character, its own architectural style, it's own mix of businesses. Finally, there's a street-level, small-scale neighborhood level of organization within the named neighborhood, which reflects the character of the larger unit but can vary significantly from street to street. Different people, different buildings, differently particulars of architecture, but still part of the larger whole. On the borders, things can blend together a bit, or the divisions might be sharp and unmixed.
The point is: New York lends itself very handily to random generation. Write up a set of tables for each neighborhood to generate the buildings on a given street and the people you're likely to meet, (with a sub-table for the qualities of the local subway station) and your players can wander wherever they want without running out of city. You'd want to define the major buildings by hand -- pick out spots for things like the Statue of Liberty, Central Park, and the Metropolitan ahead of time -- but the rest of it could be done on the fly.