OpenSourcery

/**
 * Implementation of hook_context_conditions().
 */
function mymodule_context_conditions() {
  $items = array();
  // The key used here will be used below when setting the context.
  $items['mymodule_foo'] = array(
    '#title' => t('Field Foo Value'),
    '#description' => t('Set this when field_foo has a certain value.'),
    '#options' => array(
      // Note, these are hardcoded here for simplicity, but could
      // easily use something like CCK's content_allowed_values() api
      // function for more dynamic population.
      'value1' => t('Label 1'),
      'value2' => t('Label 2'),
    ),
    '#type' => 'checkboxes',
  );
  return $items;
}

function mymodule_nodeapi(&$node, $op, $teaser, $page) {
  if (isset($node->field_foo[0]['value']) &&  $op == 'view' && $page && menu_get_object() === $node) {
    // Use the same key here as used to define the context condition
    // above.  Note, this logic would need some re-working if
    // field_foo allowed multiple values.
    context_set_by_condition('mymodule_foo', check_plain($node->field_foo[0]['value']));
  }
}