MaintenanceMode by Peter Burlingham

Allows you to put your site into maintenance mode so that users who are not logged in are taken to the login screen and a message is displayed accordingly.

This is a little module that allows you to put your site into maintenance mode should you ever wish to.

The module checks to see if you're a superuser and if you're not then every page on the site which you try and load will send you to the login page with an appropriate maintenance message.

The styles are hardcoded into the module at present, so changing the default style of the front-end EDIT button could well have a knock-on effect.

Thanks to adamkiss whose ListAfterSave module this was written over the top of ;)
Thanks to ryan for solving the last piece of the puzzle and helping me display the message in the top-left of every page.