I found this while researching another problem and thought I would post it so it would be in the search database:

Q282102 - PRB: ServerXMLHTTP Object Adds Content-Type Header With XML Data. Applies to: Microsoft XML, Version 3.0

When you post a DOMDocument using the ServerXMLHTTP object, a "text/xml" header is added automatically. If you set a request header, you will get an duplicate content-type header ("text/xml, text/xml") which may cause a problem.

Workarounds:

Don't call setRequestHeader when using the DOM, pass DOMDocument.xml.