In CCK version 6, non-required radio button fields add an extra "N/A" option. This is a deliberate decision made by the CCK module maintainers -- see http://drupal.org/node/368771 for discussion -- but sometimes this is not what you want. Here's a helpful snippet that can be adapted to remove this extra option on CCK radio button fields for your site.

/**
 * Implementation of hook_elements().
 *
 * This extends optionwidgets_elements() to add in additional processing.
 * Note that your module must be weighted higher than optionwidgets
 * for this code to take effect.
 */
function example_elements() {
  $type['optionwidgets_buttons']['#process'][] = 'remove_radio_na';
  return $type;
}

/**
 * Unset the N/A option on option widget radio buttons.
 */
function remove_radio_na($element) {
  unset($element['value']['#options']['']);
  return $element;
}