soluzione

codice:
function load_content(popup)
{
    if (popup)
    {
        $(document).ready(function(){ tb_show("text",popup); });
    }
}
codice:
<link rel="stylesheet" type="text/css" href="/thickbox.css"/>
<script src="/jquery-1.1.3.1.pack.js" type="text/javascript"></script>
<script src="/thickbox.js" type="text/javascript"></script>
<script src="/global.js" type="text/javascript"></script>

<select name="test" onchange="load_content(this.value);" style="width:160px;">
    <option value="">Where are you going?</option>
    <option value="TB_inline?height=150&amp;width=400&amp;inlineId=myOnPageContent_1">Rome</option>
    <option value="TB_inline?height=150&amp;width=400&amp;inlineId=myOnPageContent_2">Florence</option>
  </select>

<div id="myOnPageContent_1" style="display: none;">
      

Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome Rome </p>
  </div>

  <div id="myOnPageContent_2" style="display: none;">
        

Florence Florence Florence Florence Florence Florence Florence Florence Florence  </p>
  </div>