RFC 1630, written in 1994, specifies news: URLs that can be
used to link to a news article, and notes that they're unusual because they
don't specify where to retrieve the article from.
Now that Message-Id search engines (such as Google Groups) are common and
index both newsgroups and mailing lists, I frequently find myself wanting to
cite a news article or mailing list post; a messageid: URI schema
would be useful for this, and the user agent could use a configurable
Message-Id search engine.
In the future, being able to use this with the HTML cite attribute
might actually offer an advantage to using semantic HTML in email.