Generates a custom menu sub menu nav using wordpress wp_get_nav_items(). See the following for details: http://stackoverflow.com/questions/11935423/how-do-i-generate-a-custom-menu-sub-menu-system-using-wp-get-nav-menu-items-in-w/11948374#comment19273552_11948374

gistfile1.php

<?php
  $menu_name = 'main_nav';
  $locations = get_nav_menu_locations();
  $menu = wp_get_nav_menu_object( $locations[ $menu_name ] );
  $menuitems = wp_get_nav_menu_items( $menu->term_id, array( 'order' => 'DESC' ) );
?>

<nav>
<ul class="main-nav">
    <?php
    $count = 0;
    $submenu = false;

    foreach( $menuitems as $item ):
        // get page id from using menu item object id
        $id = get_post_meta( $item->ID, '_menu_item_object_id', true );
        // set up a page object to retrieve page data
        $page = get_page( $id );
        $link = get_page_link( $id );

        // item does not have a parent so menu_item_parent equals 0 (false)
        if ( !$item->menu_item_parent ):

        // save this id for later comparison with sub-menu items
        $parent_id = $item->ID;
    ?>

    <li class="item">
        <a href="<?php echo $link; ?>" class="title">
            <?php echo $page->post_title; ?>
        </a>
        <a href="<?php echo $link; ?>" class="desc">
            <?php echo $page->post_excerpt; ?>
        </a>
    <?php endif; ?>

        <?php if ( $parent_id == $item->menu_item_parent ): ?>

            <?php if ( !$submenu ): $submenu = true; ?>
            <ul class="sub-menu">
            <?php endif; ?>

                <li class="item">
                    <a href="<?php echo $link; ?>" class="title"><?php echo $page->post_title; ?></a>
                    <a href="<?php echo $link; ?>" class="desc"><?php echo $page->post_excerpt; ?></a>
                </li>

            <?php if ( $menuitems[ $count + 1 ]->menu_item_parent != $parent_id && $submenu ): ?>
            </ul>
            <?php $submenu = false; endif; ?>

        <?php endif; ?>

    <?php if ( $menuitems[ $count + 1 ]->menu_item_parent != $parent_id ): ?>
    </li>                           
    <?php $submenu = false; endif; ?>

<?php $count++; endforeach; ?>

Add (array) in foreach( (array) $menuitems as $item ).