For those who wish to see samples of the actual data files used in creating the USPS zip code and 2006 ZCTA boundary maps, there is a sample directory available in both shapefile and SQL data dump format:
The SQL data dump is PostgreSQL format, so no geometry is involved, however, the spatial data is stored as point and polygon data types so spatial queries can still be performed. This is for those that do not have PostGIS or another shapefile aware data system.