Dante5, все элементы должны иметь параметр name
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
<form>
<input id="xb" name="test" value="12345" type="hidden">
<input name="a" type="checkbox" value="3">
<input name="b" type="checkbox" value="1" checked="checked">
<input name="c" type="checkbox" value="7" checked="checked">
</form>
<script>
var s = $('input:checked, #xb','form').serialize();
alert(s);
</script>
</body>
</html>