http://docs.jquery.com/Plugins/Validation/Methods/required
edit the code for 'select' as below for checking for a 0 or null value selection from select list
case 'select':
var options = $("option:selected", element);
return (options[0].value != 0 && options.length > 0 && options[0].value != '') && (element.type == "select-multiple" || ($.browser.msie && !(options[0].attributes['value'].specified) ? options[0].text : options[0].value).length > 0);