This is an oldy but goody for which there's a large need.
I manage a community news site that, when a new post appears, send a notification to a "sister" Google Group e-mail list site. The members of the email list respond with emails to the topic that should be, or become, comments to the original post.
This was discussed once before, at: http://wordpress.org/extend/ideas/topic/respond-by-email-integration-with-comments-system
But perhaps the time has come. Wordbooker offers this type of back-and-forth integration with Facebook articles & comments. And it seems as if it should be possible with email threads, even if some manual linking is required.
P.S. This is one I (and others) would gladly pay for.