Руководство jQuery

    function countChecked() {
      var n = $("input:checked").length;
      $("div").text(n + (n == 1 ? " is" : " are") + " checked!");
    }
    countChecked();
    $(":checkbox").click(countChecked);

"HTML"

<form>
    <input type="checkbox" name="newsletter" checked="checked" value="Hourly" />
    <input type="checkbox" name="newsletter" value="Daily" />
    <input type="checkbox" name="newsletter" value="Weekly" />
    <input type="checkbox" name="newsletter" checked="checked" value="Monthly" />
    <input type="checkbox" name="newsletter" value="Yearly" />
  </form>
  <div></div>

"CSS"

  div { color:red; }

"Живой пример jQuery"

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
						"http://www.w3.org/TR/html4/loose.dtd">
	<html>
	<head>
	<link href="http://test-drupal.ru/themes/slyweb/css/jqueryiframe.css" 
	rel="stylesheet" type="text/css"/>
	  <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.5.0/jquery.min.js"></script>
	<script>
	  $(document).ready(function(){
	
    function countChecked() {
      var n = $("input:checked").length;
      $("div").text(n + (n == 1 ? " is" : " are") + " checked!");
    }
    countChecked();
    $(":checkbox").click(countChecked);


	  });
	</script>

	</head>
	<body class="iframe">

	
<form>
    <input type="checkbox" name="newsletter" checked="checked" value="Hourly" />
    <input type="checkbox" name="newsletter" value="Daily" />
    <input type="checkbox" name="newsletter" value="Weekly" />
    <input type="checkbox" name="newsletter" checked="checked" value="Monthly" />
    <input type="checkbox" name="newsletter" value="Yearly" />
  </form>
  <div></div>


	</body>
	</html>

	
	<style>
	  div { color:red; }

	</style>

Метод появился в jQuery 1.0 (http://api.jquery.com/checked/)

Версия jQuery 1.4.2 Документ создан 2010-08-21