The problem:
to display specific CCK fields on a simplenews node form depending on the newsletter that has been selected.

Background to the problem:
I have many different newsletters, and each newsletter will contain different CCK fields for different purposes. To enhance the UI when creating a new newsletter issue I only want to display the appropriate fields for the newsletter that has been selected.

For instance:

-- If ‘General Newsletter’ is selected, the following fields field_general_mainarticle, field_general_mainevent are to be displayed on the form.
-- If ‘UK Newsletter’ is selected, the following fields field_uk_jobs, field_uk_activities are to be displayed on the form.

I have tried adding #ahah properties on the newsletter vocabulary in hook_form_alter (when form_id = ‘simplenews_node_form), but then I just get totally stuck as I don’t fully understand ahah.

Any advice would be welcome.