使用custom walker. 扩展功能start_lvl() 和end_lvl():

class WPSE39005_Div_Walker extends Walker_Nav_Menu
{
    /**
     * @see Walker::start_lvl()
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @return void
     */
    public function start_lvl( &$output, $depth )
    {
        $output .= \'<div><ul class="sub-menu">\';
    }

    /**
     * @see Walker::end_lvl()
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @return void
     */
    public function end_lvl( &$output, $depth )
    {
        $output .= \'</ul></div>\';
    }
}

<?php
// If there is no menu assigned we show nothing.
has_nav_menu( \'top-menu\' )
// Path to the file with the walker relative to theme’s root.
and locate_template( \'php/class.WPSE39005_Div_Walker.php\', TRUE, TRUE )
and wp_nav_menu(
    array (
    \'menu\'       => \'top-menu\'
,   \'walker\'     => new WPSE39005_Div_Walker
    )
);
?>