I general I don't think it would affect AC as a forum. Until they deem a forum like AC to be "suspicious" and they could easily lock it down. That's the big problem here... if the bill is passed they can lock down anything they think is "suspicious" on whatever reasons. Yet, there have been some things regarding SOPA I've read that this law could lockdown any site that has copyrighted material linked; hence youtube clips. Most of em are copyrighted (if you're going the music route), they're just not enforced that hard. But the "what are you listening to" thread suddenly becomes a cesspool of illegal activity for SOPA.
SOPA in general is stupid, they're applying business law on matters of art. I rather think there needs to be an overhaul in the entertainment industry instead of applying law and train of thought that worked 30 years ago.
Also; SOPA just makes things harder for people to learn about new media. I kinda wonder that if this bill would get passed, then... in one day, we'd be back to "buying everything" which isn't really a gradual approach people would deal with that easily. Besides, mentality of said people might be a problem as well. People don't even think it's wrong to download stuff you'd have to pay for, and they apparently need a law to tell people that it is wrong. In a lot of cases it takes a really, really long time for people to accept such laws and deal with it, especially a law like this where the modern world is a swirling vortex of media and influence all around, that for most is acquired for free.
Imagine this; independant filmmaker X makes a movie, of an idea that he got after he accumulated a bunch of movies and books of the internet for free (or; illegal). The amount of influence he's getting out of these media is something he'd never get if he had to pay for all, thus in a way you're dumbing down creativity IMO.
I can see that doing arts is something that needs to be viable just in case to make a living, but for what it's worth, if you feel you cannot make art cause you have the urge to make large sums of money out of it, you're not doing it "right" in the first place. Art in my humble opinion shouldn't be a vessel to harvest millions of dollars. If that's someones idea of art and he doesn't make that amount, stop making it and get a different job. But yeah, that's probably me, as I always liked the idea of making art from "nothing".
To be honest I'm kinda disappointed that only a few (relatively speaking) websites join in. For what it's worth they could've pulled the plug on the entire internet for 24 hours. Yeah, companies would have a financial setback, especially those that are depending on the internet, but if there's laws in the making where they can exclude YOU as a company from the party, I'd worry a bit if my company would be viable if I'm not online, because such scenario might be possible for anyone.
Dolby; maybe this will give you some more information, it's from the site I host my artwork at, and they started a debate and they shortly explain what the entire deal is about, especially for artists and such
DeviantArt SOPA debate