Tried that, worked ok during upgrade from 2.5 to 2.6. But do not log out after you have activated maintenance mode. I did (to test the screen) and I had trouble getting back in – even http://www.example.com/wp-admin/ showed the maintenance screen. I renamed the MM plugin folder on the server and got back in. When logged in, I renamed the MM folder back and disabled the MM screen in the correct way.
