I ran "yum upgrade" early yesterday from a remote ssh login, but upon looking at the set of packages I decided to wait until I could sit at the console to reboot. So I declined to proceed with the upgrade when prompted. Later when I ran "yum ugrade" again, it found no packages to update. After some poking around checking the mirrors, running "yum clean headers", etc., I finally tried "yum clean all" and thereafter "yum ugrade" worked flawlessly. I've never seen this happen with "yum update", so I don't know why it would be different with "upgrade", but there you are.