It would be wonderful to be able to create and edit a wiki page via email. I think this is a feature of Basecamp, and I've heard of a Mediawiki plugin to do this.
Our use case is that a lot of information that logically should be in the wiki is exchanged via email, and only some of it ever makes it to the wiki. If getting that information to the wiki were as easy as forwarding the email to a particular address, or including that address in the group involved in the email exchange, it would happen much more often. The system could even add tags that it thinks are probably appropriate.
If it's to be used as an email address in the discussion, rather than just getting the email forwarded at the end, it would have to handle both usenet-style quoting and Microsoft/corporate-style quoting, updating the wiki page appropriately.
For bonus points, it would be great if edits to that wiki page generated emails to the email participants, based on the concept that the wiki page is simply one of the participants in the discussion, but with a better and more public memory.