Well, putting the extension in explicitly doesn't seem to change anything, and when I enter the URL of the XSL sheet (without the extension) in Firefox, it finds the file okay.

Also, changing the MIME type of the document to application/xml doesn't fix it either, so I doubt this is because it's an XHTML document.
Right now I'm using the text/xsl MIME type for the XSL sheet, is this correct?