Last week I was updating a CCK custom field from Drupal 5 to Drupal 6. The D6 module updates are pretty simple, the catch is that CCK migrated to version 2 with D6. Unfortunately, the documentation for the CCK API is not as complete as the API documentation for Drupal core.
I got stuck on some of the naming, specifically around formatter functions. There's a blog post by Jennifer Hogdon that really helped sort out all the confusion: http://poplarware.com/articles/cck_field_module