I get the whole notion, that the best you can hope for in relating an IP to a specific entity is, most narrowly, a computer, or, more broadly, a network. I wonder what the practical effect of the ruling will be?
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!).