Developer info (D7+) - Creating a text field with Wysiwyg enabled

Drupal 7 introduces the form element #type text_format, which is a text-format-enabled version of a textarea.

  // Retrieve the default values for 'value' and 'format', if not readily
  // available through other means:
  $defaults = array(
    'value' => '',
    'format' => filter_default_format(),
  );
  $my_richtext_field = variable_get('my_richtext_field', $defaults);

  // Just construct a regular #type 'text_format' form element:
  $form['my_richtext_field'] = array(
    '#type' => 'text_format',
    '#title' => t('My richtext field'),
    '#default_value' => $my_richtext_field['value'],
    '#format' => $my_richtext_field['format'],
  );