Liebe(r) siGGi,
da habe ich doch bei eigenen Recherchen was für Dich gefunden:
http://developer.apple.com/internet/webcontent/xmlhttpreq.html
Da heißt es unter anderem:
Note: It is essential that the data returned from the server be sent with a Content-Type set to text/xml. Content that is sent as text/plain or text/html is accepted by the instance of the request object however it will only be available for use via the responseText property.
Liebe Grüße aus Ellwangen,
Felix Riesterer.