The basic premise of Bittorrent Copyright Infringement cases is that plaintiff content holders obtain the IP addresses of numerous infringers by participating in torrent swarms, then ask the court for permission to subpoena ISPs to give subscriber information about who was using that IP at the time of the download.  Other problems with the “copyright trolling” model aside, let’s take a step back and consider the ramifications of having nothing but an IP address to implicate an infringer.  Here are four reasons why we need to be extremely wary about equating an IP address with a person:

1. Open Wifi – There’s at least one unsecured connection in my apartment building.  Some people set up their wireless routers years ago, before connections were more encouraged to be secured.  Others prefer the convenience regardless of the risks, or are simply unaware of the risks.  Downloaders of infringing or illegal material understandably like open Wifi because it is much harder to track their activities back to them.  A judge in a Bittorrent case cited a snafu where the FBI traced downloads of child pornography to a family’s home and performed a raid, seizing their computers.  Later, they discovered that a neighbor had been using their open Wifi to perform the downloads.
2. Ability to easily bust even secured Wifi – That same case notes that the child pornographer was also able to access a secured State University of New York Wifi network.  It does not require substantial skill at this point to steal someone’s Wifi password.  Lifehacker has tutorials explaining how extremely easy it is to crack WEP security, and the stronger protocol WPA is not that difficult either. Ultimately, it is reasonable to suspect that any Wifi network has been compromised at some point.
3. Multiple users of the same IP – I share my Internet connection with roommates, and we let our next-door neighbor use our connection when hers was down.  Whole businesses, schools, or families can be logged on to the same IP at the same time.  The person who pays for the connection and the ISP has on file for subscriber information is not necessarily the one who downloaded the file.
4. Proxies – Sophisticated Bittorrent users use proxy connections or VPNs, accessing the Internet through one or more intermediary servers to hide their tracks.  BTGuard specifically markets itself as a proxy service that caters to users of Bittorrent.

On the downside for the content owners, not equating people with IP addresses means that establishing that a certain individual downloaded a certain file at a certain time is going to require serious forensics.  On the downside for the downloaders, though, judges might consider the number of legal hoops the content owners must jump through to obtain a judgment, and entertain larger awards.