>Where do you look up latitude and longitude based on zip code? Cause a zip code could be a huge land area, or just a single campus...

Jason,

There is no relationship between zip codes and longitudes and latitudes. For example. Here in Georgia we're south and east of Chicago. Zip Codes here begin 30. Milwaukee which is north and as approximately far west of Chicago has zip codes beginning 53. Chicago begins with 60.

I did find the following link that might be of interest:

http://link-usa.com/zipcode/

They have products that claim to "calculate" the distance between two zip codes. However, I'd guess that they use a look-up table to get the longitude and latitude of the two zip codes and proceed from there.