I don't have a problem with this type of invasion of privacy as long as there is leniency and descression. I think if the ISP kept the record and a court order needed to be issued, it would probably help them catch the bad guys moreso than hurt the good guys.
On the other hand, if it leads to the FBI busting people for going to porn sites or downloading a song or 2, the FBI agents should be arrested for waisting our tax dollars.
As far as email traffic etc., I have always treated emails that I have sent out as relatively public anyway. I think the ones mostly against this kind of thing are the bleeding hearts and the ones who have something to hide.