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 pippo(aSelect) {
 var a = new Array();
 //
 l     = aSelect.length;
 j     = 0;
 for (i=0;i<l;i++) {
  curOpt = aSelect.options(i);
	if (curOpt.selected) {
	 a[j] = curOpt.value;
	 j++;
	}
 }
 document.getElementById('tx').value = a.toString();
}
//-->
</script>

</head>
<body>
<select id="sel" multiple="true" size="10" onchange="pippo(document.getElementById('sel'))">
 <option value="1" >Uno</option>
 <option value="2" >Due</option>
 <option value="3" >Tre</option>
 <option value="4" >Quattro</option>
 <option value="5" >Cinque</option>
 <option value="6" >Sei</option>
</select>


<textarea id="tx" rows="5" cols="50"></textarea>
</body>
</html>