It has already begun. Inc case you haven't heard, Wikipedia will not be available today, January 18th 2012, what is to be "Protest SOPA and PIPA Day" of sorts.
Other major sites, such as Reddit, TwitPic, Mozilla or Boing Boing are following suit.
And Google is still available, but they make the point, too:
What's it about? Watch…
PROTECT IP / SOPA Breaks The Internet from Fight for the Future on Vimeo.