FYI: http://www.iana.org/assignments/message-headers/prov-headers.html
IANA is quite reluctant to actually remove things from the registry. As long as nobody implements them I suppose this is ok as well :-) Coming Wednesday we can register the new headers unless some discussion suddenly starts to happen on the IETF headers mailing listthat needs addressing...
PS: I haven't yet reviewed what Jonas and Arthur have suggested. I will try to get on that soonish.
-- Anne van Kesteren <http://annevankesteren.nl/> <http://www.opera.com/>