Less than ten minutes of research leads me to a very effective offline password editing utility; about an hour later (returning to the problem after dealing with some lab problems and after a bit of fussing to actually create the bootable CD) I've successfully cleared my forgotten password and can boot up my machine (and set a new password).
Fortunately, I remembered the hard disk password, or I'd have been out of luck, because the utility would not have had permission to read or write the security files.
So, campers, here is why this post is public: if you don't have a hard disk (BIOS) password set on your machine, *anyone* can use this same tool to hack your machine. It works with everything up through Vista, apparently (although it's a bit trickier there).
Here's a simple New Year's resolution: take the time to set the passwords on your hard drives.