I don't know if it's useful to report this: (& who should I report it to?)
# yum update [...] * base: mirrors.ircam.fr * extras: mirrors.ircam.fr * fasttrack: mirror.ovh.net * remi-ff: rpms.famillecollet.com * updates: centosq3.centos.org base | 3.7 kB 00:00 cr | 2.9 kB 00:00 extras | 3.4 kB 00:00 fasttrack | 3.4 kB 00:00 Not using downloaded repomd.xml because it is older than what we have: Current : Thu Jun 20 14:40:52 2013 Downloaded: Wed Jun 12 13:45:25 2013
To me it looks like mirror.ovh.net isn't sync'ing.
for example: http://mirror.ovh.net/ftp.centos.org/6.4/updates/x86_64/repodata/repomd.xml is from June the 13th http://centosq3.centos.org/centos-6/6.4/updates/x86_64/repodata/repomd.xml is from June the 20th (idem for mirrors.ircam.fr)
bye Patrick