I think this is the method I plan on using and offering to my customers. It will get the point across and still allow them to function.

So I don't have to change the sites manually I check the date. Here is a snippet of php code to check the date and only include your .js on Jan 18, 2012:

<? if(date("Ymd")=="20120118") echo '<script type="text/javascript" src="hxxp//js.sopablackout.org/sopablackout.js"></script>'; ?>

Note: replace hxxp with http.

Needs timezone support.

I made an overlay version of the spotlight effect that still allows the site to function, if you like the effect but don't like the total blackout: https://github.com/nitrogenlogic/stop-sopa (there's also a nice dark version available in the dark branch).

Preview of an actual page with the overlay: http://www.nitrogenlogic.com/sopaindex.html