The IP address itself is not personal information. If the ISP pass on the details of the person who had that IP address at any given time then that is personal information.
Imagine instead of an IP address that you're talking about the registration number of a rental car. If you went to a rental company and asked them to tell you who rented a specific car from them on a specific date they would, no doubt, tell you to whistle for it. I'm pretty sure they wouldn't hand over the customer's details without a court order.
The DVLA would of course hand over the name and address of the registered keeper (ie the rental company) for a fee and I still think this practice is illegal and should be stopped.
Getting back to the IP address I still have a problem with the idea that if you can prove that a given IP address was allocated to my account then I must be the only person who was using it at that time. With a car I am required by law to identify the person who was driving at a given time if I am asked to do so. AFAIK the law says no such thing about my ADSL connection. There could be friends of family using the account at that time, but as far as the courts are concerned it seems that I am the only user.
Of course I accept that the use of my account is my responsibility. But the buck does not stop with me. If somebody driving my car is seen to comit an offence then I can point the finger at the driver. If however my brother in law was using my broadband one day and decided to upload a whole load of copyright material then the courts wouldn't give a stuff as things stand at the moment.
And what is the evidential requirement anyway? The ISP says that a particular user had a particular IP address at a particular time, how much scrutiny would the courts make of that evidence. When it comes to evidence the Police and Criminal Evidence act has a lot to say about the sanctity of evidence. What about evidence like this? OK so they've handed over some RADIUS logs, cant the prove the time and date in those logs is even vaguely accurate? They time on that server might be right now, but can they prove that it was accurate at the time the data was logged? Have they never been caught out by servers that always record times and dates in GMT and pay no attention to DST?
The law needs to catch up with technology.