Yep, it was the hard drive
I finally had some free time tonight so I put an old 2.5-inch hard disk mechanism I've been hanging onto into my 17-inch PowerBook. You may recall that I started having problems with it last week -- it would lock up and go very slow, then freeze, and finally couldn't boot at all. Sometimes it would show up in FireWire Target Disk Mode but I could never get any data off it before it locked up the bus.
Putting this ancient 40GB mechanism cured the PowerBook's ills -- it's booting and working like a champ. So I guess all I have to do is buy a new drive for it. Judging from the stock at NewEgg, It seems my choices are to either spend a moderate amount of money buying a drive that's the same capacity (80GB) but works a hell of a lot faster (7200RPM) or spend a lot of money buying a drive that's moderately bigger (100GB) and moderately faster (5400RPM).
Advice? Space was always at a premium with the old drive, but now that I've parted company with it, there's probably a lot of cruft I realistically don't need to reinstall.
Also, disassembling the 17-inch PowerBook is an incredible bitch. It's got the tiniest screws imaginable every which way, weird little ribbon cables that you have to hook your fingernails under to work loose, and is generally one of the most user-unfriendly systems I've had the displeasure of working on since my Bondi Blue iMac last needed an upgrade.
Also, the amount of vile stuff trapped in my keyboard was also unimaginable. Turning it over and shaking it a few times, I had the displeasure of seeing a steady blizzard of crumbs, skin flakes and God-knows-what-else fall to the floor. Using a tiny screwdriver and scraping delicately between the keys, I also managed to wedge loose some furballs that looks like they'd been hacked up by the cat.