There's an interesting debate between Dan Solove and Bruce Boyden on one aspect of this -- limits on discovery of an anonymous poster's identity -- in this Concurring Opinions post and the comments to it.

I've cowritten pieces on related subjects before (injunctions in copyright cases and independent appellate review in copyright cases), but I'm not sure how things should play out here, where literal copying is alleged, and where the procedural protection is the emerging procedural rule related to limits on the outing of anonymous posters. My inclination is to say that the procedural rule should be applied, but the requirements of the rule should be easy to satisfy (since the rule generally requires only showing of a serious prima facie case that the speech is unprotected by the First Amendment, something that's usually pretty easy to do with file-sharing of literal copies of others' songs). But I can't say I've looked at the subject closely.