Prior to doing this, typing in the field would fire autocomplete as expected. And I also see the communication in the console of firebug. Having made the two changes, I see the new path in the correct place in the form field, but nothing happens when I type in the field other than the characters appearing...the logic isn't fired and firebug shows no communication.

I also tried registering the original autocomplete path with a different callback. Same result.

Below are the functions.

/**
 * Implements hook_menu().
 */
function my_article_menu() {
  $items['a/categories'] = array(
    'page callback' => 'my_article_autocomplete_callback',
    'page arguments' => array(1),
    'type' => MENU_CALLBACK,
  );

  return $items;
}
/**
 * Implements hook_form_alter().
 */
function my_article_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'article_node_form') {
    $form['field_a_categories']['und']['#autocomplete_path'] = 'a/categories';
  }
}