The IP addr argument is just the barristers throwing everything they can think of at the case. It won't hold up as stated. It is fair to say that the IP "owner" (for example you or I in respect of the IP addrs given to your home routers) is not necessarily the copyright infringer. Could be someone else like a family member. Doesn't make the IP addr inadmissable as evidence.
The likely outcome of it will be very similar to vehicle reg marks - just because you're the reg'd keeper doesn't necessarily make you liable (in criminal proceedings at any rate), but if you can't provide a reasonable explanation to the court of either who the driver was, or why you couldn't have been the driver ("I was in Australia at the time m'Lord - here's my passport") you can expect the court to hit you with a bag of hammers.
Also note that in civil proceedings the burden of proof is MUCH lower (51/49 as opposed to 99.9% in criminal) If the judge thinks you were more than likely the person responsible you are munged.
"...the owner of a network being responsible for the action committed to the outside world from it"
Ummm, I can't quite get my head around the semantics of that one. It appears you are comparing computer misuse with copyright infringement. There are some similarities. Directors are responsible for copyright infringement AS WELL AS the corporate body that actually committed it if it was committed with their "consent or connivance" (love that phrase). However there are differences insofar as re: copyright infringement both civil remedies and criminal sanctions are available.
"...they have no obligation to identify perp users"
I wouldn't try that argument in court - you can be summonsed to court as a witness, and the court can compel you to answer questions that are asked of you. Failure to do so = contempt = time in clink. As a board member are you going to be in contempt of court to protect your wayward employee, or just name the idiot that caused you this grief?
I don't see the ACS argument working (they allowed their network to be used to infringe copyright) - the wording of the legislation just doesn't fit. You'd have to interpret words in ways other than their everyday meaning, which the courts are loathe to do without good reason. However, as I said above, if the ISP cusomers who are linked to the IP addrs don't have a good explanation as to how it wasn't them on the PC, given it's a civil case it's not asking too much of the court to find as fact that they were in fact the responsible party. Just depends on the DJ - some are soft as they come, others not so...
It may be possible to argue that they committed the infringement by negligence (not taking reasonable steps to secure their PC/network) - it's very common for the "guilty mind" to comprise either intent or negligence. They could also compare the situation to landlords whose premises are used to deal drugs / illegally imported fags etc.
However, it very much appears that nobody really wants this to go to court if they can help it. The problem is, if it becomes widespread "knowledge", everyone on the interwebs says "oh just ignore that official looking document", it may well go well for a while, but in the end the rights holders may feel obliged to take action to protect their rights. How much would it suck to have been told by all and sundry "ignore it" then you are the one where the DJ nails you for £5k because 1. you infringed copyright; and 2. you were unreasonable in ignoring the pre action corres?
Don't expect the gravy train to continue indefinitely - the rights holders hold a lot of sway, and the fact is sharing copyrighted material without a license *IS* an infringement.