Hi all,
I've just noticed that the release of Fedora Core 6 has brought the Fedora download site, and half of its mirrors, down. Installing Virtualmin on Fedora systems requires a working yum, with OS and updates repositories...and while these sites are down, yum won't work. I'm not sure exactly what to do about it, except wait until the Fedora site is back online.
If you happen to find a working mirror, you might alter your /etc/yum.repos.d/fedora.repo and /etc/yum.repos.d/fedora-updates.repo files to switch the mirrorlist= line to a direct baseurl= pointing to your working mirror.