I slapped this extension in the "Debug" position (last position on my template), and it worked like a charm. I checked it using several different IPs and then mine, and it does what it says: blocks everybody but up to five different IPs.
You can customize the logo, the offline message, and set which pages should be hidden (including "All").
My only problem is that the Sitename isn't centered along with everything else on the offline page. And the site's template background is still visible, but not a big concern. A great tool that should have been included in the Joomla! core, in my opinion.