Google has taken a subtle approach in participating in the SOPA Strike. When you go to the main page, it shows a sentence “Tell Congress: Please don’t censor the web!” with a link to another page.
Following the link, you will be led to a page where you can see Google’s information about SOPA and PIPA. On the right hand side, there is a form for you to fill in your particulars to take part in the petition.
As for Wikipedia, a screen will greet the user with the sentence “Imagine a World Without Free Knowledge”
Lets hope that the fight is not in vain. Reddit will be joining in a few hours time (there is a countdown counter at their site)