[matthewsimon]

> ... the most densely populated zip codes ...

It turns out that zip codes aren't as populous as you might imagine.

As of the Y2K census, only a dozen zip codes have over 100,000 residents -- 80% of all zip codes have less than 15,000 residents, with the median zip code at 2,500.

(Source: http://www.census.gov/geo/www/gazetteer/places2k.html )

[greenyoda]

Some zip codes have zero residents -- a few large office buildings have their own zip codes, but nobody actually has a home address there. Examples: the Empire State Building, the Pentagon, the former World Trade Center, IRS tax processing centers.