We completed building and programming a custom Internet Caching router based upon OpenBSD.  We were able to bridge two NICs and cache Internet traffic using Squid.  On my tests we saved about 100mb of downloaded content in two days using two computer.  That's about 50mb per day per computer in a test environment.  On our production network we should see between 2 and 5 GB per day savings.  Wow- that will help!