[herrold@centos-5 yum.repos.d]$ cat CentOS-RPH.repo
[53]
name=CentOS-$releasever - 53
baseurl=ftp://xps400/pub/mirror/centos/centos-qa/centos/5.3/os/i386
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
[herrold@centos-5 yum.repos.d]$
Without it, as noted in the twitter feed, we get some security updates from 5.2 [avahi, nss, avahi-compat-libdns_sd and nss-tools], and a unsolveable update set.