jQuery uses XHR's responseXML property to get the XML document from an XHR request. However, IE6 and IE6 will only work if you request an XML document served as text/xml or application/xml. A content type of application/atom+xml won't work, for example. You can see a demo here:

http://brh.numbera.com/experiments/browserdemos/ie-load-atom.html

The jQuery docs mention a workaround: http://docs.jquery.com/Specifying_the_Data_Type_for_AJAX_Requests

Since I use jQuery primarily to isolate me from these browser defects, it would be nice if jQuery could work around this as suggested. So if it was in IE and the content type wasn't a simple XML type, you could instantiate and fill a Microsoft.XMLDOM object on behalf of jQuery clients, transparently working around IE's issue.