The most they'd ever do is a stylesheet change or an overlay on top of the text. Actually removing their content, for even a day, would irreparably screw up their search traffic in the future. Any site that big is being constantly revisited by Googlebot.
I wonder. I run a website that has ranked on Google page 1 for the relevant search terms just behind Wikipedia for years now. But it's been on the backburner for some time; I've basically been ignoring it. A couple months ago a server migration that escaped my notice took it completely offline. Totally dropped off Google. Well, I just updated the DNS record a few days ago and within a day or two it was back on page 1, right behind Wikipedia.
So in my experience Google is remarkably forgiving.
If Google wanted to show support against SOPA, have they considered telling their bots to grant everyone amnesty on Jan 18 for taking down their content? Merely blacking out the color scheme of your site is great for awareness, but it doesn't do much to put you in the experience of what it means when a website is taken down.
Conceivably, we could serve the normal site to search engine spiders and black it for everyone else. However, we have a complex load balancing setup that may make this impossible.
An all-black theme would do the trick, but redirecting using Temporary Redirect shouldn't screw up search if we do it right.
Your best option would be a 302 redirect to a SOPA-specific page. Google won't follow that and it shouldn't screw your search engine rankings. At least, that's how I understand it.
I wouldn't really want to count on that; Google tends to guess what common response codes mean. The safest response would probably be to use "503 Service Unavailable" with an error page.