This is a bit late, I know, but I found a similar setup described at:
http://www.gurulabs.com/goodies/YUM_automatic_local_mirror.php
Their server appears to be down right now, but that's where I found the information. The original was designed for Fedora, but it was adapted to CentOS pretty easily. I've been using this for about 4 months now with great results.
Rather than spoofing the mirrorlist.centos.org entry in DNS, I just have anybody that wants to use the local mirror put an entry in their /etc/hosts file pointing to the IP address of the local server. The Perl CGI script adds the entry for our local server to the results returned from the real mirror list and depends on yum-fastestmirror to pick the local server from the list.
Since we don't mirror all architectures on our local server, I added some intelligence to the Perl script so that it would only add the local server if the requested repository, release, and architecture were on the local server. If anybody wants to see the final results, just let me know and I'll post my modified script and the Apache config fragment or a pointer to it.