Apparently you can opt out if you write to their office in Los Angeles within 30 days. If you don't agree to the new clause and waive your rights, you will not be able to sign in to PSN, and are effectively banned until you agree.
Also apparently, the Supreme Court made this kind of thing accepting in a ruling a few months ago. Interestingly, the same practice is already everywhere:
It has become routine now that when someone opens a bank account, subscribes to a cable TV service, buys a cellphone, a computer or a new car or makes a purchase online, he or she agrees to let disputes go to arbitration.
Many employers include the same kind of fine print for new hires, blocking class-action suits for employees with discrimination or wage complaints.
The ability of members of the general public to gather together to sue companies is being hacked to pieces. People may still sue individually, via arbitration, with all of the costs that entails, but because of those costs and the lack of collective force, people will simply be discouraged from suing altogether.