OK, I may be a clueless bastard that believes fairy tales and does not comprehend the dangers and law of unintended stuffs and so on, but...
1) Storage costs for ISPs. GIVE ME A BREAK! Lets say a record is 100 bytes per event (and it won't be more than that since they are only asking about DHCP records), lets say an ISP has a million users and they all get new IPs twice a day (lets pretend no cable modem is ON 24/7 for months and same IP is not reassigned to the same MAC at reconnects). Such ISP would end up with horrid 73G file a year. It would likely compress to 10% of that size with zip but lets say we keep it uncompressed. Puhlease, ISP that can't handle THAT doesn't deserve to live.
2) Public AP. Same math- different budget or providers but I can guarantee that simple firmware update for the router and retention policy "download once a week, burn on a 30c CD and put it into a safe" or something like that can be easily implemented.
3) Home users... Plain stupidity, don't even want to go there but router developers should start taking better care about securing this stuff by default, like forcing first password change and defaulting to WPA2.
There were few other points floated: text files are editable, MACs are spoofable, etc. All valid, but nobody said that logs alone are supposed to carry burden of proof. Making part of evidence chain might be enough.
Finally, privacy concerns. First, bull about "ISP would have to store mount of PII" is just that, bull, unless bill says "and will not issue such IP before driver's license or alternate form of ID is provided". Just MACIP mapping. If whoever asks for these logs can find machine with that MAC- they have something. ISPs can also (and IMO should) include customer ID number or something. Now, there is (IMO) a very legit worry about censorship and about authorities using this to inquire "how went to this Arabic newspaper site" or "who googled 'how to make a nuke from household materials'". I think this is a HUGE concern and we might want to concentrate on that and how it will play with Patriot act and such. But rest is noise (again, IMO, please correct me)