Yea, that is a good idea and I am actually working on something similar. I have downloaded the zipcode coordinates from the Tiger census data at:
http://www.census.gov/tiger/tm s/gazetteer/zcta5.txt
Also the geocoders here:
http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html
and here:
http://www.geocoder.us are helpful to latitude and longitude points.
With this information, I can actually outline the entire zip code property lines in Google maps using their polyline function.
I got the ideas from all the brilliant developers at
http://groups.google.com/group/Google-Maps-API