http://www.sil.org/sgml/xml.html#xmlMediaTypes
Which, in addition to providing all kinds of useful background, pointed me to 
an Internet Society Internet Draft on "XML Media Types" by E.J. Whitehead and 
Murata Makoto. 
ftp://ftp.ietf.org/internet-drafts/draft-whitehead-mime-xml-04.txt
It's an excellent draft, and makes clear that character encoding issues need 
to be addressed in transfers.  The only question I have is that they're 
registering text/xml and application/xml.  Are we expecting all XML-encoded 
files to transfer under these two MIME types, or will more specific MIME types 
emerge for particular types of XML documents?
Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies