Please upgrade to 2.3 here.
The 2.2 and earlier branches are no longer being updated.
Child categories (aka subcategories)
The admin UI doesn't seem to offer nested categories, but class.categorymodel.php contains a lot of relevant cogwheels. Is this something that's likely to be enabled in the near future ?
We are considering moving a medium sized phpBB 3.x board (45k users), I could definitely help with the migration script and probably a Drupal 6.x bridge. The other thing we need to look into is an ajaxy way for users to filter out categories they are not interested in.