Obviously a Major Malfunction (Updated)
Apparently my backup of the site doesn't include backups of the templates.
Unfortunately, a misbehaving plug-in appears to have overwritten my main index template. I've re-installed the default, and will be working on restoring some semblance of normalcy to the site.
Okay, I managed to get everything more-or-less back the way it should be. Please let me know if you run across something that isn't working right.