WWW::MapBlast sounds like a good starting point. This module will give you the latitude and longitude of a given zip code. Next step would be to use a module like GD to apply the zip codes. I would try some kind of World to Viewport algorithm to do so. It has been a long time since i did any graphics programming, last time was in Java. You can find my explanation on World to Viewport here (which is paraphrased from the book Computer Graphics).

Remember also that jcwren has already done this something very similar at the Big Monk Map.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)