Well, that was a nightmare. I realized only after finishing a long entry and logging out for the night that my attempt to put in per-category access controls earlier completely broke access for any unauthenticated users. It still looked fine to me, of course, since I was necessarily logged in when I activated the new module, and then seeing some errors in the logs and not wanting to deal with them immediately, deactivated the module again.

A good chunk of time later (in which I would have much rather been sleeping), I appear to have gotten basic access working again, though I'm now stuck with the taxonomy_access module in a partially working and unremovable state.

What a wonderful thing to have to deal with two days before going on vacation. I could have fixed it by restoring from the previous night's backups, but then I would have lost the long entry I made this evening.

I'm not sure what the moral of the story here is, except that perhaps for all the wonderful things that Drupal provides, using the contributed modules is still a very risky business, and you shouldn't try anything new on the same day that you're doing any writing. I filed a bug report on the Drupal site for those interested in how this progresses.