Another data point:
I updated 2 machines, one a P3-UP machine and the other a P3-SMP machine. Both were fully updated with the CentOS 4.3 errata releases.
I did this on each machine:
yum clean all yum -y update yum yum clean all yum upgrade
No errors or warnings (beyond a few config files installed as .rpmnew) were seen during the upgrade process. I rebooted immediately after the upgrade was complete. In the 3 hours since upgrading I have seen no problems with the upgraded software.
Good job, Johnny!