Hi!
The documentation for *rewrite-for-session-urls* says that ""This only happens, though, if the body's content type (see CONTENT-TYPE) starts with "text/html""".
XHTML can have other content types, too. See http://www.w3.org/TR/xhtml-media-types/
OK, application/xml and text/xml are a bad choice, because you can't say if it's really a kind of XHTML or just another XML application. You have to parse the content itself for that.
But application/xhtml+xml is OK and recommended.
Regards, Stefan