I think there isn't a separate mobile URL for the login page.
It seems that the user agent is determined only after logging in.
There is a mobile view and a desktop view, but the login page looks the same for both: mail.opera.com/m gets me to the same page as mail.opera.com if viewed in the same browser:
In all the browsers I have on N900, I get taken to mobile view when I log in (likewise, desktop view in all desktop browsers), but the URL is one specific to my account, not a generic one. Would it help if I gave you that?

Anyhow, I believe all the info publicly available for this site/service is here:
http://my.opera.com/mailteam/blog/

I asked in the blog comments to find out more, but I suspect I made it a stupid question to someone who knows the answer...