Are customers liable?
By accepting the Terms and Conditions and giving your ISP permission to intercept your communications you may actually be opening yourself up to criminal liability under RIPA.
As mentioned a multitude of times, consent is required from all parties for the interception of communication; by communicating with someone else with the knowledge that there is going to be an interception without the consent of the other party(ies) you could be deemed as complicit. All sort of cans of worms could be opened such as aiding and abetting; conspiracy and entrapment.
You could also be opening yourself up to Copyright Infringement offences such as Secondary Infringement and Vicarious Infringement. BT et al should be reminded that Copyright Infringement becomes a criminal offence where commercial gain and profit are involved; and since this is a profit based system (the ISPs get a cut of the advertising revenue) it seems to fall under criminal copyright law.
I am not aware of any case law in the UK which covers these points explicitly (but that doesn't mean it doesn't exist) however, there is case law elsewhere in the world. If I remember correctly there has been at least one case lost see:
Kelly v. Arriba Soft Corporation (336 F.3d 811(CA9 2003))
It should be noted that even in the case of Perfect 10 vs Google (which was originally judged in favour of the Plaintiff (Perfect 10) and then overturned on appeal) Google only managed to get the the ruling overturned on Fair Use arguments. Fair Use arguments don't work in the Phorm situation because there are differences. Google Images only created a derivative works in the form of a thumbnail which then linked directly back to the websites they came from. Phorm is copying the entire page using an illegal wire tap, so I don't think they could use the same arguments of Fair Use.
http://www.jurpc.de/aufsatz/20020029.htm (in German sorry)
which basically covers the situation regarding caching of websites in Europe with regards copyright law and reinforces that it is actually Copyright Infringement under European Law.
The above stemmed around Archive.Org (aka WayBackMachine) and the courts accepted that the Plaintiff had a case for the court to hear with regards breach of contract, based on the Terms and Conditions she had on her website which were breached by Archive.Org when they cached her pages.
Obviously Archive.Org settled out of court so no judgement was ever received, but they did acknowledge the infringement in their press release.
My advice to website owners who do not wish to have their pages intercepted and copied by Phorm systems (or indeed any other such systems) would be to add some Terms and Conditions to your website explicitly refusing the right to copy the pages and would then be covered under copyright law, contract law and RIPA as I understand it. If the Home Office want to try and throw around the implied consent argument, then it cuts both ways. Phorm accessing the website are bound by your Terms and Conditions through the same implied consent and would therefore be in breach of contract should such terms as "Phorm may not access or copy this website under any circumstances" appear in those terms. So potentially, a lot of popular forums could make a boat load of money from suing ISPs and Phorm for Copyright Infringement and Breach of Contract and even possibly bring criminal charges since the infringement is being used for commercial and financial gain.
Even if there is a slightest chance that my statements above are correct, they are reason enough alone, not to allow the interception of your communications.
So in the words of Nancy Reagan "Just say No!" [to Phorm]