I'm gonna take a wild guess & say you're running Red Hat Linux 9. Did you check this sticky thread?
http://www.linuxquestions.org/questi...threadid=89379
Shortly after RH9 was released, Red Hat updated their certs. This broke up2date until you applied the fix. (
https://rhn.redhat.com/help/latest-up2date.pxt)
If you ARE running RH9, I'm not sure you're gonna be able to pull updated packages since it is no longer supported.
You should probably take a look over at
http://www.fedoralegacy.og for patches to "end-of-lifed" Red Hat releases.
-Jake