EFF has released Open WiFi and Copyright: A Primer for Network Operators.
Here's an excerpt from the announcement:
At EFF, we are big fans of open wireless. But we also know that operators of open networks sometimes worry that they could be legally responsible if people use their networks to engage in copyright infringement. We've put together a short white paper that generally explains the scope and limits of operator liability for the acts of users, and additional steps network operators may choose to take to further limit their legal risk. As we explain in the paper, copyright liability for the acts of your users is less likely than you might think, as long as you (1) simply provide a means of transmission; and (2) act reasonably and responsibly.