Wiki page on SOPA (read the discussion of DNS filtering, especially about the Pakistan/YouTube episode, around footnotes 30 to 32. The Pakistan/YouTube episode (allegedly) occurred when a single Pakistani DNS server was deliberately mis-configured so as to ‘blacklist’ YouTube in Pakistan, resulting in world-wide outages for YouTbe for several hours.
Prof Zittrain’s TED Talk (the whole thing is worthwhile but beginning at approximately the 5:00 point, Prof Zittrain begins discussing how the DNS works, as an intro for his discussion of the Pakistan/YouTube episode)
The point being that the most serious criticisms against SOPA (imho) reatle to the DNS filtering mechanisms that may have unintended consequences. The Zittrain piece helps me (at least) begin to grasp the technical background of the debate.
I think that trademark owners need enhanced remedies (you know I’m a notice-and-takedown fan), but breaking the Internet would be a bad side effect.