No I don't believe a brute force would work ether for the same reasons. And anyone that thinks of PS3 cloning to get back on if they get banned, please forget it, its not ever going to work.
I don't believe Sony will ban IPs, but think its more likely to be Mac address lock outs, maybe with some hardware information for more accuracy in the banning process, like motherboard version and S/n, the IBM call's S/n, user account details, plus hdd0 file allocation table, BDVD info, the list is long.
One thing is for shore, is they will change a zero to a one on your PS3, and stop loads of other things for ya, in the process when blocking that PS3 for life. There's an old saying " If I cant have you, then no one can"
But seems that there is something else. IF someone manages to get a console ID from someone else... the COMBINATION of the ID and the USER is never the same ... so normal people do not have to worry i guess?
Well, the end of this will be a PSN account for each PS3 in the next firmware. No more multiple accounts, only one, with your e-mail and password attached to your ps3.
And that will blow that Cloud thing they were doing about taking your save files to your friend's house by entering your psn login and password in your friend's ps3.
Anyway, sony is screwed, one way will ban "innocent" consoles another way will remove another feature that is multiple account (but sony is very good removing things, hun ?) and that way will kill Cloud app.
If you are a victim and its a mistake, contact them and explain, they will let you back on again if your reasons are just. And like Barry said they don't allow anyone on the network without joining in some way, even if you quick connect your still in one way agreeing to their terms, like age and location, then it activates the PS3, so it knows were you live to give you local TV stuff and so on. In the process it has your IP address, and consoles information ready to trace back to your isp if needed.
But don't worry about the ISP side because 9 times out of 10 your isp will tell them where to get off, unless you've hacked them or terrorists etc. or until Sony enforce a court order on them.
Your ISP does not hand out user information to anyone, unless as I said they have a court order I believe, and then and only then its down to them how far their archives go back.