Is there really an obvious 1st-amendment issue here? SOPA is a censorship tool with great potential for abuse, but as written and intended it targets IP infringement, a very narrow category of speech that our society has already decided to censor. The 1st amendment will come into play when Congress wants to censor more, not merely censor better.
Prior restraint is the biggest 1st-amendment issue possible (as expressed by SCOTUS in several opinions), and shutting down sites based on a say-so and a court order that the site "looks infringing" is prior restraint.
If I'm not mistaken SOPA can take down websites that merely link to infringing sites. I believe that is very dangerous for the Internet, because it's what it's built on, and it's also very dangerous for freedom of speech. Linking is speech.
