Out of interest, is this something that is always required when upgrading Grub?
i.e. should one always manually run "grub-install" /dev/XXX" after doing so?
No, it shouldn't. But in my case, I didn't "upgrade grub". I simply did a "yum update" which installed, among other things, a new kernel, so a reboot was needed. And when I rebooted, grub wouldn't start. So I booted into rescue mode from the CentOS CD #1, and did a "rpm -e grub kernel" followed by "yum install grub kernel". This did not fix the problem. I had to boot into rescue mode again and do a "grub- install /dev/sda". The big question remains, what happened on my system that corrupted the MBR. I could have been anything at all since the last time I rebooted (about a month ago) and not necessarily the "yum update".
Alfred