The MPAA has already figured out how to stop it: they are getting the IPs of individual users and going to their ISPs and demanding that they be shut down if they keep using the service.
Geeze thats a lot of computers to shut down!
I've also heard that some of these file sharing progs are basing themselves out of the US, often in rather dumpy countries, where stopping this sort of thing is very low on the priority list.
RIAA will lose this, as we've known for years. You can't fight the future. You have to work with it. Its a hard lesson for them but in time they'll learn. Shutting Napster was a step forward in their efforts, but they're simply walking on a treadmill which is going faster than they're walking!