A bit more thinking reminded me I'd seen this:
http://www.theregister.co.uk/content/28/33661.html
I also remember some article on there about some guy proposing some sort of secure authenticated email system, but I can't remember any details or find the article.
The problem with any form of secure, authenticated email system is it destroys the simplicity of the current setup. At the moment, anyone can set up an SMTP server anywhere and send some mail with no need for connections to other services, safe in the knowledge that assuming network conditions are OK etc, it will just work.
Once you start messing about with authentication (think SSL certificates) then it suddenly becomes a lot harder to set up a mail server.
Of course, some people argue this is the whole point and would help stop spam etc, but for the average administrator it'd be one more pain in the arse.