I'm fairly certain that deep packet inspection would be required for an ISP to connect customer to IP address to visited resource. That being said, it would be trivial for an ISP to log the date and time each customer held a given IP address. If a plaintiff were to provide that date and time information, it seems at least technically possible to link that activity to a subscriber account.
What I don't see though is how you prove that the person listed on the account was the one performing the activity. Then again, all it takes is convincing a few people (6 for a civil trial jury?) who weren't clever enough to get out of jury duty.
What I was implying is that a snapshot of the peers in a given torrent is not exactly hard evidence that each of those peers has a 100% downloaded copy of the files. For example, I remember that some torrent clients have a "super-seed" mode, where they appear as a normal peer with less than 100% completion. I know this sounds a bit like "the dog ate my homework" excuse but it sets a bad precedent of what constitutes an infringement - is it downloading a whole copy, uploading a whole copy, or just a part (unusable by itself) of the work?
Well, what's a worse precedent? Being technically innocent of infringement since you skipped a single bit on the end of a download or the court ignoring your intent? Intent has been the cornerstone of many copyright infringement defenses. It would seem a bad move to claim that intent is irrelevant at this point.
What I don't see though is how you prove that the person listed on the account was the one performing the activity. Then again, all it takes is convincing a few people (6 for a civil trial jury?) who weren't clever enough to get out of jury duty.