See: http://apt-mirror.github.com
Added functionality for downloading installer section of repo
e.g in mirrors.list will download amd64 versions of netboot and cdrom install media
deb-installer-amd64 http://archive.ubuntu.com/ubuntu precise netboot cdrom
deb-installer-amd64 http://archive.ubuntu.com/ubuntu precise-updates netboot cdrom quantal-netboot quantal-cdrom