I read somewhere, perhaps even on el-reg, that provided an overview of how Phorm works. IIRC whenever you browse, phorm asks your browser for your phorm cookie, this contains your unique identifier so it can work out your habits from your previous sessions.
So, how about just deleting your cookies. Sure, you won't be "opted-out" whatever that means, but you also won't get ads targetted to your browsing habits, they'll have no data to go on. . It's not like googlemail isn't doing something similar when it displays your email. There's no privacy objection there is there? I'd quite happily sign up to phorm if they, I dunno, gave me an ISP discount of 50% for opting in, there wouldn't be any privacy brouhaha then would there, you're getting a benefit for signing up.
Every one has their price, you just have to decide what yours is. I am currently a BT customer, and their service has been reliable enough to keep my custom. There are plenty of technical work-arounds for phorm (Tor?, VPNs and the like) so I'm not bothered.