For me the most worrying aspect of this is the precedent it sets: that ISPs can send information on your browsing habits to a third party (and in this case a highly dubious third party) without your permission.
The opt out that is offered only stops you getting targeted adverts - it does not stop information about your browsing being sent to Phorm. Despite their protestations, I cannot believe that they will be doing nothing with all that data - why would they incur all the costs of receiving and processing it if they are not going to use it in some way?
How long before this is forced on BT subsidiaries and, if the ISPs can make money out of it (as I believe there must be or else why would they be doing it), many other ISPs adopt the same system?
Because of the precedent it sets I'd urge everyone to sign the petition - not just users on the three ISP's named (how many other ISPs are already in secret talks with Phorm?).
http://petitions.pm.gov.uk/ispphorm/