Search form as list item in menu?

You can add any item you want to a menu by filtering wp_nav_menu_items.
From the tutorial How to Add a Search to Menu, this is how you’d add a search form:

add_filter('wp_nav_menu_items','add_search_box', 10, 2);
function add_search_box($items, $args) {

        $searchform = get_search_form( false );

        $items .= '<li>' . $searchform . '</li>';

    return $items;
}