As a matter of fact, that's how my configuration is right now. All http traffic gets redirected to https.
I'd like to support a better solution in which ssl would be used only when needed. That would result in better caching at web proxies on ISPs and companies and woudn't prevent intrusion detection and prevention systems from working.
This kind of behavior will probably come by default in mailman 3. In the meantime, I guess it should require only small changes on the code to generate relative urls instead of absolute, and if it's as easy as I think it is then it would be a nice improvement in mailman 2. :)