Just found out that in CentOS 5.3 (and perhaps in upstream RHEL too) the automatic update yum-updatesd is not working anymore. There is a hint in the Release Notes.
The concerning bug report speaks of an minor issue. In my opinion its a major issue! Automatic updates do not work anymore and you don’t even get notified about that fact!
Update:There are some scripts out there which check for updates without the need for a special daemon. One of them is yumnotifier.