Today I decided to install all the latest updates (including the -06 kernel). The "yum update" seems to have run just fine (no errors), but when I rebooted the system it just sits there with the word "GRUB" in the upper left hand corner. I booted from the CentOS 4.5 install CD into rescue mode, did a "chroot /mnt/ sysimage" and reinstalled grub and the latest kernel, but it still gets stuck in the same place. It's not that I can't boot the new kernel; I can't even get to the grub screen!
I'll answer my own question. In addition to re-installing grub, a "grub-install /dev/sda" was required to get the system to boot. I thought that reinstalling grub via yum/rpm would take care of it, but apparently it didn't. Lesson learned, panic averted.
Alfred