I have a subdomain based multisite set up. Without Domain Mapping activated, I can log into http://www.maindomain.com, and switch to network admin, or any dashboard e.g. site1.maindomain.com and am always logged in.
If I enable Domain Mapping, even without mapping any domains, no matter which settings I choose in Domain Mapping, if I log into http://www.maindomain.com then go to another site's dashboard, I have to log in to every site individually.
If I start at site1.maindomain.com, I can navigate to http://www.maindomain.com and be logged in, but other sites bring up the login screen.