Maybe the disk is dying? Did you run smartd (it requires -d ata for SATA disks; this option needs to be put in smartd.conf)?
It's a brand new disk (well, less than three months old), and it pretty much did it from the get-go (as did the previous disk). I've replaced the SATA cable and updated the system BIOS (it's a Lenovo ThinkCentre M51, and I am using the on-board SATA controller) as previously suggested. I was hoping that the errors logged with syslog would help uncover the root cause. It just so happens that the first time after I configure syslog to log to another system, the disk becomes unbootable.
The error messages could also indicate bad cables.
I already replaced the cable, and this is is an intermittent error.
I would boot from the CentOS 4.3 Live-CD, and take a look at the disk with smartctl. If the disk is indeed dying, I'd try to save its contents to a fresh disk, using ddrescue. Unfortunately there are 2 programs with this name (http://www.garloff.de/kurt/linux/ ddrescue/ and http://www.gnu.org/software/ddrescue/ddrescue.html); I have very good results with the latter - don't know if it's on the LiveCD (if not, it should!).
Great idea, booting from the CD as I type this. I had tried booting the install CD in rescue mode, but that resulted in a kernel panic when it tried to mount the disk. Let's hope I have more luck with the LiveCD.
Alfred