As mentioned over a week ago, and then promptly ignored by me, there's a post script to the story about my PowerBook G4. It's not dead. The disk isn't even dead. It's back in my office, admittedly not doing much, kind of sulking at being shunned for the faster, slicker MacBook Pro, but it's working. Apparently, although the problem was originally replicated (the machine would not boot for the technician), it spontaneously started working again. The technician "walked off to get a cable" (presumably to try booting it from an external disk), and on his return the PowerBook was showing its login screen. It booted five more times for him, and it booted again when I got it home. I have no doubt it still contains a fairly sick disk, but it's alive for now.
There's even more good news. Next Byte Glenunga refunded the entire $75 diagnostic fee. Frankly, I was not expecting this. Clearly, neither were the staff in their retail section on a Saturday morning, as actually getting the $75 took upwards of 15 minutes. The whole place seemed to be staffed by work experience kids during what must be one of the week's busiest time slots. The rate-limiting step for my transaction seemed to be finding someone who knew how to operate the EFTPOS machine.