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I'm not sure that they would all do it, but it seems possible. Especially with Google, considering Sergey Brin is one of the more vocal opponents of SOPA. It would also be easy for Google to still allow searches, but change the Doodle that appears above the search box in the same way they do for many holidays and anniversaries. That would have far less shock value than a block out, call-to-action page, but with little negative impact on users, which would be the downside to it actually being effective. It would definitely get an avalanche of attention from the general news media if multiple sites went black at the same time to draw attention to SOPA, and hopefully the more people learn about SOPA the more they will oppose it. I don't think there would be very much negative backlash if this does occur, but I will be somewhat surprised if it happens.