The Recording Industry has decided that suing customers one at a time is not slowing things down. So the next step? Get the ISP to do the work for the RIAA. The RIAA now wants ISP's to be cops.
In my understanding of the DMCA, if you start checking the content on your system, you lose DMCA protection.
In my understanding of privacy, the RIAA (and its sibling, the MPAA) still need to get a court order for action. How does an ISP know if the letter is legit or if the sender even owns the copyright or if the IP address was spoofed etc. etc. In the story on CNET with Bayou Internet's Jerry Scroggins, the letters Jerry showed did NOT match the form letter that the RIAA said they would use (see here).
Here's a question for the RIAA: What happens if the ISP does shut down the offender? He moves to another ISP. So all you are asking the ISP to do is to lose a paying customer. That's dumb.
An analogy for you: If I shoot you, do you sue Smith & Wesson? No. Do you sue the gun store? No.
Piracy happens. Is it decimating your business? I don't see it. Did I miss the announcement that Warner Music was bankrupt or in need of a bailout? Does all of the piracy mean that you have to change your business model? Absolutely. But just like the newspaper industry, you would rather try to put your finger in the dike. Oh, well. That's your problem, not the ISP's. In cases of heavy users, your concerns may be parallel but that doesn't mean you are friends.
UPDATE: RIAA's New Policy Isn't About Deterrence, It's About Sidestepping Due Process on TechDirtUPDATE2 from ARS