Just so it's in the historical record here, Oracle's Spatial module should be helpful, assuming you've spent the big bucks on your database. You can see details of this module for both 8.1.7 and 9i (both require a free Technet login and password).
9i appears to have improved upon 8i by no longer treating all coordinate systems as being imposed on a flat Cartesian-style plane. That is, 9i actually takes the Earth's curvature into account when doing such calculations.
The downside of Oracle Spatial is that it's part of the Enterprise Edition, not Standard. That's $40k/CPU list, instead of $15k/CPU. But...if you're already using Oracle Enterprise Edition, maybe this is your ticket. It certainly has a lot of nice searching features, judging from the docs.
Incidentally, the us_zipcodes table appears to include latitude and longitude for the zip codes. I'm not at my OACS installation right now to verify that the table got created when I installed OACS.
