currently if you change $maintenance_mode in /conf/_basic_config.php from 0 to 1 it kills all access to the blog. Even your feed goes dead. What I would really like to see is some sort of advancement there. For example if you make it be 1 it should kill all access except for ID#1 accessing the back office. Change it to 2 and all registered users can get to the back office. In both cases the feed shouldn't look like your web is dead because really it isn't dead - it is in maintenance mode. So for example it replaces the real feed with a "post" that has 'doing maintenance' as the title and (of course) no content.
I've been using it a bit lately and being totally locked out has some drawbacks eh? Like, I can pretty much deal with it but it'd be way cool if ID#1 could still get in to the admin side of town ... and especially not make the feed look like you went offline.
Cuz clearly if you're doing maintenance you're not gone is the thing. And some maintenance is easier with the back office live.