Good stuff Andrew, here are a couple of other thoughts:

• imap access to the archives, readonly for most people, write access for admins
• "send me this message" link so that if you're reading an article in the archive, you can click this link to get a real copy sent to you. may not be as important w/nntp or imap access. perhaps integrate with an email comment (i.e. 'send message-id')
• we talked about perhaps have a tinyurl-like alternative access to the message (though this probably isn't the definitive url)
• integration w/digester so we can have new kind of "summary-with-links" digest
• if we use the original message's message-id, we need to have a policy for dealing w/collisions