Ok, booting in Single User mode did show the mount as read-only. Issuing "fsck /dev/disk0s3" produced the exact same error output as I quoted earlier about the super-block.

How do I go on from here? I do think (from a quick google) that the correct fsck command is /sbin/fsck_hfs for HFS disks though. Let me quickly give that a try...

EDITED/ADDED:

Using the hfs fsck worked - at least it says it checked and repaired. I had to also specify the "-f" flag to force the operation. It first detected it was a journaled filesystem and didn't bother to check it.

A check with Disk Utility still reports an error. "Invalid directory item count" it should be 827 instead of 829. That is the same problem fsck repaired except it said it was repairing a should be 825 instead of 827.

Now I need to go get some breakfast, but I'm starting to prepare myself for having to open up the machine and replace the drive.