I was having a conversation with z00dax on IRC, and the suggestion was made that I post about this to the mailing list:
CoBlitz is a nifty free distribution service for large files over http, that utilizes PlanetLab's network of hundreds of machines to provide bittorrent (or better)-like speeds while offering many of the advantages of bittorrent (less stress on mirror sites).
The website is: http://codeen.cs.princeton.edu/coblitz/
To use it: ..."You add the prefix http://coblitz.codeen.org:3125/ to the URL you want to serve, and CoBlitz does the rest"...
I'm sure you could do something like use tcpwrappers to limit the machines that can get to the mirror URL to the PlanetLab network.
There was a good article on CoBlitz in the August 2006 edition of ;Login: - http://www.usenix.org/publications/login/2006-08/pdfs/park.pdf - however you need to be a Usenix/Sage member to read it (t becomes free August 2007).
Another idea was to use a binary diff program like bsdiff - http://www.daemonology.net/bsdiff - to distribute patches to a base beta CD (that suggestion was less popular with z00dax - "thats why god made mirrors" :-)
Oh, and someone might want to update the html files at http://dev.centos.org/mirrorscripts ; they reference torrents for really old beta versions.
Eagerly awaiting CentOS 5 public betas,