We've released a new version of our USPS Zip Code tiles this week, they are the same data as the old version (01/2008) but are more detailed, better formed, faster and prettier.
The old version used a method that required each tile to be projected into the "World Mercator" projection on the fly, this caused some stretching of the image and some smearing at higher latitudes. It also caused more drag on performance as the processor needed to change the image fit the tile. In addition, there was one image group for every two zoom levels, which would lead to pixelation.
The new version has one image group for every zoom level from 5 to 14, zooms above 14 interpolate the image from zoom 14, so there is some pixelation at very high zooms. The new version also uses an improved method of load balancing, and as stated, is pre-projected into "World Mercator" for both faster processing and better image quality.
The new load balancing method uses the modulus 3 of the "x" tile to compute which tile server to use, the old method simply had different zoom levels on each server. The old method was more efficient for server disk space but resulted in slower performance, especially in IE. With the new method, all three servers are sending tiles to each client, which is much more efficient for server processing power and bandwidth. The resulting increase in server disk space turned out to be irrelevant.
The new version can be found as a link on the old version's page for now. http://www.usnaviguide.com/zip.htm