
When digging around for Drupal Forms API examples make sure you know which version of Drupal the examples you find are for; the differences between 4.x and 5.x are just subtle enough so as to be terribly confusing.
Finally, I found this discussion thread. For a clear, functional example see the second comment down by pwolanin, “Wrong code above, and 2nd point: form_id”. The code works, and the explanation of how the form_id is named is tremendously helpful: