function columbit_menu_zindex(menu_parent, zindex){
    menu_parent.each(function(){
        jQuery(this).css('zIndex', zindex);
        if(jQuery(this).children('ul').length > 0){
            jQuery(this).children('ul').css('zIndex', zindex+1);
            columbit_menu_zindex(jQuery(this).children('ul').children('li'), zindex+1);
        }
    });
}

jQuery(document).ready(function(){
    var first_level = jQuery("#menu-main").children('li');
    //columbit_menu_zindex(first_level, 1);
});