If an IP is suspected of criminal activity, and it can be related to only a particular network/house, the case may not be a slam dunk. However, it may well be enough to create all sorts of joy for that network/house. It's probably enough probably cause for a warrant which will then find all computer attached to that network confiscated. If they can pin the behavior to a specific machine, then anyone who had access to that machine could be under suspicion.
If you run an open WiFi site, the net could be broadened to any computer the the radius of your signal--the neighbors will love you. Even though one can make the arguement that some random car with a laptop might have parked in front of your house, and you might carry the day in court, you'll still have a lot of hassle and legal fees.
(And that doesn't include anything that might shake out from what's on those computers (child porn, etc.), discovered as a consequence of the search. For that matter, how are charges filed for such a thing on a shared computer?)
I'm not sure the RIAA would be able to get such warrants/subpoena based solely on an IP address. While it will prevent them from simply creating suits based only on IP address, at the end of the day it's just one more hoop to jump through.
At the end of the day, that's my point: while it definitely raises the bar for legal action, I'm not sure it does much more than that in practice.
Note that I'm not a lawyer--this could be 100% bunk (or more!).
That said, one line in the original post struck me. "You can buy it from Amazon as a used book, but that isn't your preferred medium." What if a book you wanted or needed were only available as a hard copy, and is currently produced? For the sake of argument, let's say it's brand new, hot off the press--only Stallman might argue that the author doesn't have a right to a copyright. It's not your preferred medium, but it is legitimately available. There is a pirated copy available. Would you take the pirated copy? Would you argue that, since they aren't making it in your preferred format, it's fair game?
Take it to the next level: say, a year or two from now, your book is released--brand new printing. Perhaps even expanded! Alas, no etext. Would you buy a copy, just to bring you back to good standing, or would you rationalize it by saying there is no etext (your preferred medium, after all)?
(I'll grant this: in any scenario, while perhaps not legally correct, I would see no ethical problem obtaining a legitimate copy of the book, and grabbing the etext. My comments are more around just going out and grabbing the etext.
What! Rich Stallman is against something that he didn't personally conceive of and name? I can't believe it! He is usually an open-minded individual, willing to foster and encourage new concepts, and never one for not-invented-here syndrom!
"Money is the root of all money." -- the moving finger