I don't see the big issue with storing IPs as long as the only way to getting the data is through a court order. My problem is that I don't trust most ISPs not to hand over the data to the police if they simply show up and ask for it "in the interest of the children/national security/etc". Also, the data should be stored for a relatively short amount of time (the EU data retention directive called for a period between six months and twenty-four months, that's way too long).

Of course, when you hear about how the French DGSE was getting raw data from Orange anyway, it's clear that it's not the police overstepping you should feel the most worried about, it's these agencies whose entire purpose involves breaking the law.