Also beware of where you leave your car and certainly, don't leave anything at all in it. If you are doinng Pen-y-Fan, then I would only recommend parking at story Arms opposite the outdoors centre, as here there is a lot of movement of people and traffic. Having said that, it still won't be "safe". Sorry to sound a bit paranoid but having seen so may break-ins lately and personally knowing lots of victims of car thefts from this area, just be sensible.
As Tony said, its quite difficult to wild camp near the main paths, but with a bit of imagination, you will be OK.