It would appear that Wikipedia's action has been joined by others, including some outside the US. According to this morning's Yahoo News, Canada's Tucows is "on strike" and offline for several hours in support of the protest, as are a great many smaller sites.
One of the most interesting supporters of this (the protest, that is, not SOPA) was one who benefits from copyrights, author John Scalzi. He's shut down his blog for 12 hours today, commenting that while he fully expects to be compensated for his work, SOPA is definitely the wrong way to accomplish that. As he put it, you don't catch a burglar by carpet-bombing an entire neighborhood.