How about it?
It could be restricted to Zen users, could speed up downloads and save Zen having to to route traffic to other ftp sites and repositories.
Zen could mirror the popular distro's such as Ubuntu, Fedora, Debian and Suse. I would put in a plea to include gNewSense as well as it is one of the few 100% free software distributions and I use it :-)
Any downsides? Well, someone would have to set up the mirrors, but I can't think of anything else particularly.