Ajax loading of kml works in webkit/gecko/presto; not in IE

Ajax loading of kml works in webkit/gecko/presto; not in IE

I'm developing yet another kml viewer for google maps, as neither GeoXml or nor GGeoXml provide a feature I really need: good dynamic network link support. The current code I've got works well in every browser except Internet Explorer, which fails when trying to load the kml.
I've set up a simple example at  http://tsr.no-ip.org/jqkml.html, although this is stripped back to the bare minimum to highlight the problem.
The problem lies in the mimetype handling. If the mimetype for the kml file is over-ridden at the server to "text/xml" or "application/xml", IE will load it without problems. However, if the mimetype is set correctly to "application/vnd.google-earth.kml+xml", it fails to load. The "+xml" format for xml-derived mimetypes does appear to be widely used, and would be worth supporting.
I located what appears to be relevant code at line 4962 of the full source of jquery-1.4.2, although appending the kml mimetype to the accepted types does not fix the problem.
If anyone can show me a way to get round this using xhr, I'll be very grateful.