Puoi fare cosi...

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function checkAll(aValue) {
 if (! aValue) {
  document.getElementById('sel').innerHTML = 'Seleziona tutti';
 } else {
  document.getElementById('sel').innerHTML = 'Deseleziona tutti';
 } 
 aColl = document.getElementsByTagName("INPUT");
 for (i=0;i<aColl.length;i++) {
  curObj = aColl[i];
	if (curObj.type == 'checkbox' && curObj.name == 'checkEmail') {
	 curObj.checked = aValue;
	}
 }

}
//-->
</script>

</head>
<body>
<input type="checkbox" name="checkAll" onClick="checkAll(this.checked)"><span id="sel">Seleziona tutti</span>

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

<input type="checkbox" name="checkEmail" value="miovalore">

</body>
</html>