function mytheme_tinymce_config( $init ) {
    $valid_iframe = 'iframe[id|class|title|style|align|frameborder|height|longdesc|marginheight|marginwidth|name|scrolling|src|width]';
    if ( isset( $init['extended_valid_elements'] ) ) {
        $init['extended_valid_elements'] .= ',' . $valid_iframe;
    } else {
        $init['extended_valid_elements'] = $valid_iframe;
    }
    return $init;
}
add_filter('tiny_mce_before_init', 'mytheme_tinymce_config');