How about being able to log in on another website which is protected via ssl?
With this scenario there would not be any clear text password sending via the internet.

forum ought to be protected by SLL 3.x and above- it is obvious

I was thinking of protecting only the log in page. The rest can be plain http.
Many hosters offer ssl with a url like http://provider.ssl/yourdomain. If you could redirect the log in page to that url and after logging in coming back to http://yourdomain, the server has to handel ssl only for logging in.