Announcing a hacker news blackout could sway the big guys to do the same.
Google have advertisers and business to worry about. They also have the G+ vs Facebook battle to be concerned about.
If Google blackout and no one else does then what will people do during the hours Google is down? They would go else where... shareholders and investors would be very unhappy. Especially if you consider these people who pop to Bing may decide they like Bing better.
If Facebook / Google / Yahoo / Bing all went offline from 6-9pm in a rolling blackout across the states on one night. (i.e. blackout by IP based on states timezone.) It would have a massive effect.
Hell, they don't even need to completely blackout. Just randomly black out elements of the page.
American's are still afraid of communism right? Lets add a bar at the top of the sites.
"SOPA would mean that we would have to black out portions of the Internet. Do you know who else does this? CHINA. Are you a communist? Campaign against SOPA here."
It's a bit direct but I thought this was how US politics worked?
As far as Google's advertisers, the G+ vs FB battle, and investors. It's either take a temporary stand now or get forced into taking a long term stand down later. What good is google ads if they can't run your ad? What good is a G+ vs FB battle if its really GOV vs G+ & FB? What good will it be to investors once their stock drops because google's ad rev (as well as other revenue sources) drops?
On the other hand, I think the GoDaddy exodus did more in a week than months full of discussions; 'big guy' corporations seem to really notice when we use our wallets to speak.
Based on that example, whom does HN control with its wallet?
Off the top of my head I think we could sway: AWS, Linode, Rackspace, Heroku and the likes -- are any of those supporters to begin with?