SOPA protests are spreading across the web and a lot of web services are planning a blackout day this week.
Google developer Pierre Far has posted an article on his Google Plus account about what you need to understand if you are going to blackout your site this week.
His basic suggestion is to use a 503 HTTP status code to ensure that Google search bots understand what is going on with your site.
503 HTTP status code indicates that the content Google bots are seeing is not the actual content and should not be indexed. This helps in preventing duplicate content penalties over a longer period of time. And it would also tell the Google search bots to slow down crawling.
Here is an older article from a Google Blog about how to handle site downtime which is similar to blackouts for a couple of hours.