I should start by saying that IANAL, however, as Duke University's DMCA agent, I've looked into the law very carefully. The fact of the matter is that you can't sue the ISP. Okay, you can sue the ISP but you won't win.

The DMCA has an explicit provision (section 512(g)(1) stating that if an ISP, in good faith, takes down or blocks access to material for which a notice of infringement was received, the ISP can not be held liable for the action. There are some caveats, but I think you would be hard pressed to prove that they had violated these very liberal caveats.

The interesting piece is that ISPs are responding to claims of infringement by taking advantage of the 512(c) safe harbor. A better safe harbor, for the ISP, would be to invoke section 512(a) which limits an ISP's liability for conduit activities, i.e. when the material is not on a computer they do now own (e.g. a gnutella client on your home machine). Presumably if an ISP invoked 512(a), the MPAA/RIAA/SBA would have to go after the user individually. Whether this is a better or worse solution for the end user is a matter for debate :)