How can I lock down my entire site?

[eluser]Aquillyne[/eluser]
My site has an admin area accessible by logging in. Inside the admin area I want a button that reads "Lock". If the admin clicks on this, every page request on the site delivers a "Site is locked down" message rather than loading (apart from the pages in the admin area, obviously). This continues until the admin clicks "Unlock".

What would be the simplest way of achieving this behaviour? I prefigure some Router extensions? Or maybe a simple hook?