Hi Joe,

The simplest way to change url, is to use set_contentUrl, however, get_iframe().src is also possible.
Note: ContentUrl should be set on server, otherwise, ContentPane will not have iframe object and there is nothing to set.
Below is example.

<ig:WebDialogWindow ID="WebDialogWindow1" runat="server" Width="300px" Height="300px">
 <ContentPane ContentUrl="http://www.google.com"></ContentPane>
</ig:WebDialogWindow>

<script type="text/javascript">
function changeUrl()
{
 var dialog = $find('<%=WebDialogWindow1.ClientID%>');
 if(!dialog)
  return;
 // instance of $IG.LayoutPane class
 var cp = dialog.get_contentPane();
 var old = cp.get_contentUrl();
 var iframe = cp.get_iframe();
 //old = iframe.src;
 cp.set_contentUrl('http://ask.com');
 //iframe.src = 'http://ask.com';
}
</script>
<input type="button" value="change url" onclick="changeUrl()" />