Well, I'm sure someone can actually bring up a class action law suit to sue over this provision in the change of terms of service if enough people notice it (which HARDOCP) does in a manner.
That being said, the courts often times favor court expediency (which class actions are not). The recent Walmart decision at the Supreme Court level rendered a decision that (while not severely) limits what constitutes a class. The plaintiffs in Walmart sued as a class of women who were passed on promotions because they were women. However, the court said the class was too large. In theory, they could probably do state-based classes to test the party size.
In this case, Sony is probably covering its ass over its recent hacking controversy. The Supreme Court (same group that decided on Walmart would likely limit the group not only to those who were generally afflicted by the outage of service, but also by any actual loss (which technically isn't quantifiable under traditional formulations but can still be determined via other remedies).
This all goes back to contract issues (not torts since there have been no actual injury). The change in the Terms of Service is a change in the contract (which it reserved for itself when you first bought into Playstation's Services. It's sneaky, but doesn't violate the original contract terms, nor is it illegal in any sense for criminal actions to proceed. With the current atmosphere, arbitration works out better for Sony because it allows them to filter out the fake cases with the real ones. If they are simply preventing ALL users from getting a chunk of the pie when they don't deserve it, kudos, but if this is an attempt to limit recovery that is deserved, then they will be in for one heck of a fight.