thanx a lot for that great plugin. I got just two things to mention:
-my Backend runs with ssl-support, so users from mapped domains got a warning, the certificate is not valid when they login (even when i redirect to the sites original domain, because the redirect-page has no valid certificate)
-I use the CAS-Plugin (http://wordpress.org/extend/plugins/cas-authentication/). When mapped-domain-users logout, they got the message that they were logged out - that's wrong. They are still logged in, because CAS got a wrong parameter (the mapped domain)
Not really trivial - isn't it? Maybe someone has thought about it?