I think this entire thread should be "expired".
It's already specified in RFC's 1327 and 2076 and no MUA supports it because it's a dumb idea. I can't believe 3 days later it's still being talked about like it's a serious thing.
Regarding the side discussion on copyrights: If anybody went into a court room and argued to the judge that they were compelled to delete an email by copyright law due to an "expires:" header they would be laughed out of the room.
