Sony once called a free PlayStation Network a "universal right" for all gamers; i.e., they'll never charge for access. But that doesn't mean they won't charge for some aspect of the service.
We see the same question at least a few times per month from readers: "will Sony start charging for the Network?" We always have the same answer: "no, but they may charge for some sort of premium service that is optional." This seemed almost inevitable from the very first moment we learned the PSN was to be free; once they were in a good position (which is now), Sony would likely provide users with an optional primo service that delivers extra content for a small fee. And now, according to this slide from Sony's Media/Investor Conference , it seems clear the time has come: right at the bottom of the PSN features on that slide, it says "New revenue stream from subscription." Now, this could mean any number of things, but we certainly don't believe you'll suddenly have to pay to play online. That will always remain free if we take Sony's words at face value; this new addition will be something special for members and if they want to pay for it, that's their choice.
You already do have to pay for certain things, like downloadable content (of course) and the online interactive magazine, Qore. Perhaps this "new revenue stream from subscription" will be a service that includes Qore and a variety of other features automatically delivered to you each month. Who knows? But we imagine we'll find out soon enough.