> ... 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 )