How to reproduce this:

$.get('/some-url', null, function(xml) {

alert('XML: ' + xml);

// Then do something with the XML.

});

Then have your controller at /some-url issue:

header('Content-type: text/xml; charset=utf-8'); // PHP Example

... and then a well-formed XML document. Your alert box won't show XML because the xml variable will be empty. Trying to do anything with the XML results in a type mismatch.

I've only tested this on FF 3.5 (which his OK) and IE7, which throws an unenlightening "type mismatch".

Sorry I can't proviode a full test case; I'm at work and there really isn't time for even reporting this. Provided 'as is'; hope it's helpful. Ignore otherwise.