Hierarchical select as radio buttons and checkboxes I have a need to display a hierarchical select of a vocabulary as a series of radio buttons representing the top-level terms.  When the user selects one of the radio buttons, the child taxonomy terms should be displayed below as checkboxes.

Do I need to build a custom AHAH form, or can I look at form altering the default hierarchical select widget?
For drupal 6 you may want to check out the module https://drupal.org/project/conditional_fields which basically lets you control which form elements depend on other form elements. It also has a handy GUI for doing this.

FYI drupal 7 has completely changed the game here and has implemented 'states api' - here is an example of what can easily be achieved - http://api.drupal.org/api/examples/form_example--form_example_states.inc/function/form_example_states_form/7