I think paperghost has a number of valid points. Using the plane analogy, determining the ID of the perp isn't as important as preventing the crime from happening in the first place (i.e metal detectors, security, etc).

In the case which the crime cannot be prevented, then tracking down the perp becomes the focus. Paperghost also has a good point here. Cybercriminals/terrorists are going to be very good presenting fake IDs and disappearing when those IDs are compromised. Regular users, however, stand to lose. Not only are massive amounts of data being collected on their personal lives, but they also stand to lose all this valuable data in one fell swoop if the ISP is compromised. Putting all your eggs in one basket, anyone?