I still can't quite believe how this turned out.
Three days ago, I was heading off to work, but wanted to load up a document in Pages before I left. I launched Pages from the Dock, but it only got as far as bouncing the icon. The PowerBook seemed to freeze up, and, out of time, I shut the lid and went to work, albeit somewhat worried. When I got home, I opened the lid, and Pages was there. Disconcerting, but I was willing to write it off to the kind of "general slowness" one observes with a two-and-a-half year old machine—the disk is slowly filling up, it stays up for two weeks at a time, and there are perpetually two users logged in, each with a handful of applications permanently open. It's been noticably non-snappy for a while.
The iCal incident later the same day, which I described in another post, apparently tipped me over the threshold for taking it seriously. The following day, I dusted off the 250G external USB hard disk which I bought almost a year ago, and followed Jamie Zawinski's advice for doing a backup with rsync.
A day and a half later, it was this morning. The PowerBook froze up again, and the disk was making some audible "I'm sorry about this, but, uh, I'm about to die"-type noises. I power-cycled the machine, and it wouldn't boot. Dead disk. Well, dead disk with a backup that's a mere 36 hours out of date.
This afternoon, I'm going to see about replacing the disk in the PowerBook, but, more interestingly, I think I'll take the opportunity to upgrade to a new 17" MacBook Pro. I'll let you know how that goes.