These cases can be defended. In fact, unless the plaintiffs find someone who can be forced to admit the download, their chances of proving the case become exceedingly small. Here are some thoughts on legitimate defenses, which you will probably find conflict with the overblown claims in some plaintiffs' demand letters:
1. The plaintiffs have to prove that it was the Defendant that violated the copyright. It is not enough just to show that the copyright was violated by someone over the internet connection owned by the defendant. If the plaintiffs are suggesting that there was an agreement between the people involved, it's up to them to prove it.
I have seen some statements, particularly in the FAQ section of the attachment that some plaintiffs send to people, suggesting that having an open wireless connection is no defense, and that it is the responsibility of the internet connection owner to secure the connection. This statement even purports to be based on legal research on the issue, and has been published as an article by a prominent plaintiffs attorney. While I cannot provide legal advice on a website, if your decision is in any way influenced by the allegation that you are strictly liable for any infringement that occurs on the internet connection that you pay for, I strongly encourage you to discuss this with an attorney, as I believe that the statement is very substantially misleading.
With that said, I have to acknowledge that it is common for the person who owns the internet connection to be related to someone who might be forced to admit to the download (See "Rule Number 1"). If the person who owns the connection knows the name of the person would have to admit to the download, that is information that the plaintiffs will (theoretically) be able to get in the discovery process. In such cases, whether the person who owns the connection was the one who actually did the downloading might not matter very much for practical purposes. However, bear in mind that if the plaintiffs begin the lawsuit by naming the wrong person, when and if they amend the paperwork to name the right person, the court could order them to pay the costs and fees incurred by the person they mistakenly named to begin with.
2. Most likely, on bittorrent networks, during their
initial investigation, the plaintiffs do not actually download the
file from the defendant's computer, as they used to do in
peer-to-peer cases over Limewire. Accordingly, the only
evidence they have to begin with is a report from a third party
server that the defendant's IP address was downloading or uploading
parts of the file. That seems to me like a very small amount of
information for the plaintiffs to procede upon, and subject to any number of objections and attacks.
3. There is no such thing as attempted copyright infringement. Ordinarily, when filing the complaint, the plaintiffs do not know whether the download was complete. Whether the plaintiffs have to show that the whole file was downloaded is subject to debate. All I can say about that is that in the old peer-to-peer cases over Limewire, the files which the plaintiffs alleged the defendant downloaded were actually found as complete files on the hard drive of the defendant's computers.
4. The law allows a court to order that the plaintiffs pay the costs of your defense if you win the case. I cannot provide any specific advice about the likelihood that a court would order that the plaintiffs pay your costs, but if I were the plaintiffs, I would anticipate the argument that if they are going to target thousands of people on the theory that whoever owns the internet connection is responsible for any copyright violation that occurs on it, the risk that they will ensnare some innocent people ought to fall on them. To that end, save every single letter, email, voicemail, and any other contact you receive from the plaintiffs.
Furthermore, I believe that this argument may be legitimately raised even in cases where the plaintiffs name Person A and then decide, during discovery (see later discussion) that they really think that Person B violated their copyright, and decide to sue him instead, which is exactly what would happen in the majority of cases I have seen.
5. There is no such thing as negligent copyright infringement. Without getting into the arguments, discussed elsewhere, let me just say that to the extent that the plaintiffs propose to prove that a Defendant is liable for copyright infringement for doing nothing more than negligently allowing someone to use his/her wifi, they are (by their own admission) asking the court to make entirely new law, which appears to be directly contradictory to the structure of the copyright statutes (See, for example, here).
With all of that said, there is one important thing to point out: unlike the criminal trials that frequently make the news and TV, in civil cases such as these, the plaintiffs do not have to prove their case "beyond a reasonable doubt." Instead, they only have to convince a judge or jury that what they say is more likely than not. Also, whereas in criminal cases, if you decide not to take the stand, that fact can't be used against you, if you decline to take the stand in a civil case (which you may or may not have the right to do), that fact can be used against you. What that means is, no matter how little evidence there is, it is a dicey proposition to go into court unless you can deny that you did what the plaintiffs allege.
|6. The Discovery Process