This tutorial explains the process behind writing a Maintenance Mode plugin for WordPress such as the one I released a few days ago: CGC Maintenance Mode. The final result of this tutorial will give you an advanced plugin capable of putting your site into maintenance mode, while allowing authorized users to view the site normally, so that you can perform updates on your theme, plugins, content, or whatever you need to do.
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You can view the plugin this tutorial is based on here, and even download it for free!
Good tutorial. I would just add output sanitization to the admin settings page: http://codex.wordpress.org/Data_Validation#Output_Sanitation
Thanks for the feedback, and you are absolutely right.