I've already composed my reply in case I receive one of these letters someday. "Dear Comcast, I am so sorry. I had no idea that copyrighted works were being downloaded via my IP address; I have a wireless router at home and it's possible that someone may have been using my connection at the time. I will do my best to secure this notoriously vulnerable technology, but I can make no guarantee that hackers will not exploit my network in the future."Link
If it ever comes down to a lawsuit, who can be certain that I was the offender? And can the victim of hacking be held responsible for the hacker's crimes? If that were the case, we'd all be liable for the Blaster worm's denial of service attacks against Microsoft last year.