I completely agree with Paul. I was a bit too quick to say "every atom element should allow xml:lang" because I wasn't aware of the fact that there are quite a few elements (rather than attributes) with completely formalized content only (such as ISO 8601 dates,...).
In many cases, such as title/summary,... it would also be very helpful to be able to mark up language changes inside an element, with something like <html:span xml:lang='...'>.
-- Asbjørn Ulsberg -=|=- asbjornu@xxxxxxxxxxx «He's a loathsome offensive brute, yet I can't look away»