It's kind of hard to figure out what to make of the state of the gaming industry right now. For the first time ever, we have 5- to 6-year-old consoles that are still chugging along. So, from Sony's perspective, what point is there to revealing a PS4? What could it do that the PS3 can't? If the answer is "not much," then there's no reason to unveil new hardware.
Long-time PlayStation veteran and current head of the division Andrew House was quoted in a recent interview as saying, "The right time to talk about new advances in hardware is when you can demonstrate a significant leap on the current experience, and something that is going to be attractive. That remains our philosophy. Beyond that we have nothing to say at this point."
This is of course uncharted territory for the console gaming market. Never before has a console lived on for more than five years with virtually no one wondering when the next one would come. The Xbox 360 is admittedly starting to show its age and there have been far more many rumors about its successor than there have been for the PS4. Don't expect a Sony announcement in this realm any time soon.