When you're logged into the blog there's a little account strip at the top of the page.
Why not add a a global application variable like $public_message.
Then add a snippet to the account strip code that publish a message when you plan to bring the site down for maintenance? If the $public_message is empty, nothing shows. But when you need to bring the site down, for any reason, you fill the variable with a message.
Then that message is shown in each session throughout WordPress.com in a prominent and hard to miss kind of way.
That way poor bloggers needn't panic when the server suddenly takes a nap. And you'd save yourself a load of aggression when users storm into the forums to demand to know what's going on... :)