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

<html>
<head>
<title></title>
<style type="text/css">
<!--

-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function selezionaCliente(val){
	alert(val)
}

function addSelect(){
var select = document.createElement("select");
select.setAttribute("name","linea");
select.className="select";
select.style.marginLeft="9px";

var option1 = document.createElement("option");
var t1 = document.createTextNode("a");
option1.setAttribute("value","a");
option1.appendChild(t1);
select.appendChild(option1);

var option2 = document.createElement("option");
var t2 = document.createTextNode("b");
option2.setAttribute("value","b");
option2.appendChild(t2);
select.appendChild(option2);

var option3 = document.createElement("option");
var t3 = document.createTextNode("c");
option3.setAttribute("value","c");
option3.appendChild(t3);
select.appendChild(option3);

var option4 = document.createElement("option");
var t4 = document.createTextNode("d");
option4.setAttribute("value","d");
option4.appendChild(t4);
select.appendChild(option4);

var divUltimoCampo = document.createElement("div");
divUltimoCampo.className="ultimoCampo";
divUltimoCampo.setAttribute("id","ultimoCampoCliente");

document.getElementById('mioForm').appendChild(select)


divUltimoCampo.innerHTML = "<a href=\"#\" onclick=\"selezionaCliente(document.mioForm.linea.options[document.mioForm.linea.selectedIndex].value);\">seleziona cliente</a>";
document.getElementById('mioForm').appendChild(divUltimoCampo)
}
//-->
</script>
</head>
<body>
<form name="mioForm" id="mioForm">
<input type="button" onclick="addSelect()">
</form>
</body>
</html>
funziona