I am using a multisite network along with the WordPress MU Domain Mapping plugin. The Hide login works great for the main site login, but I can still access the login form using "wp-admin" on any of the other sites that are using the domain mapping.
So, the main site:
http://mainsite.com/wp-admin works great!
Any other site with mapped domains:
http://mappeddomainsite.com/wp-admin still directs me to http://mappeddomainsite.com/wp-login.php?redirect_to.... with the login form