If you updated all of the blogs in your account to at least Editor level (65% have so far), then you can now choose which category you want your posts to go into.
The category you select must be appropriate for the post. If it isn't, your post will be declined by the reviewers indicating that the category does not match the post.
The categories for other people's posts will be created on your blogs on an as-needed basis. So they won't all just suddenly appear -- just the ones that are in use by the posts.
As always, let me know what you think!