If you have more than one machine behind a proxy, they will randomly pick servers from the mirrorlist and use different URL's, so you not only don't save the bandwidth by re-using the cached copy, you duplicate the copies in the cache. (Fedora already does it that way).
I created a test script to generate mirrorlists from the mirror.centos.org DNS lookup and changed yum to reference the local files that were generated as a mirrorlist= and it worked.
Now run a bunch of machine updates through the same squid proxy and see what happens. How about fixing yum to understand what multiple A records in DNS is all about?
Seems like we have the perfect opportunity here to file an RFE with skvidal for yum, and we have an intermediate fix that works in the meantime. As for the whole proxy thing, yum is known to choke on them, so it's a known issue and isn't really worth discussing on a list other than the yum mailing list.
-- Jim Perrin System Administrator - UIT Ft Gordon & US Army Signal Center