This is more some investigation for myself, since I wanted to know this. XML attributes that are in (or part of) a recommendation:
xml:lang(heavily used in XHTML documents)
preelement still exists)
baseelement still exists unfortunately)
xml:id(we should have this now, this makes live so much easier (How should the problem of identifying
IDsemantics in XML languages be addressed in the absence of a DTD?))
This is all I think, not really sure about this, but hey I have a comment function :-)
See XML and the Web essay, the wild man suggests xml:style, xml:class, xml:href, xml:rel, xml:rev and xml:script !!