Como al parecer Explorer no reconoce el atributo disabled en un option, un camino alternativo puede ser este:
Código PHP:
<!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>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Untitled Document</title>
</head>
<body>
<form id="form1" name="form1" method="post" action=""><select name="combo1" id="combo">
<option value="default" selected="selected" id="default"></option>
<option id="a" value="div1" >2</option>
<option id="b" value="div2">2</option>
<option id="c" value="div2">2</option>
</select>
<input type="button" name="Button" value="Button" onclick="o=document.createElement('optgroup');o.value=document.getElementById('a').value;o.id='a';o.label=o.innerHTML=document.getElementById('a').innerHTML;
document.getElementById('a').parentNode.replaceChild(o,document.getElementById('a'));
" />
<input type="button" name="Button" value="Button" onclick="o2=document.createElement('option');o2.value=document.getElementById('a').value;o2.id='a';o2.innerHTML=document.getElementById('a').label;
document.getElementById('a').parentNode.replaceChild(o2,document.getElementById('a'));" />
</form>
</body>
</html>