:master: non sapevo neanche io che funzionasse, ma mi pare che funzioni con IE7, Firefox2 e Opera9
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>
	Untitled Page
</title>

<script language="javascript" type="text/javascript">
// <!CDATA[


function form1_onclick(e) 
{
	if(!e) var e = window.event;	
	
	var target = (e.target)?e.target:e.srcElement;

    if(target.value == "2")
        return true;
    else 
        return false;
}

// ]]>
</script>

</head>
<body>
    <form name="form1" method="post" action="a.aspx" id="form1" onclick="return form1_onclick(event)">
        <input type="submit" name="req" value="1" />
        <input type="submit" name="req" value="2" />
        <input type="submit" name="req" value="3" />
    </form>
</body>
</html>