So the quote is from an interview, posted at Accio Quote! , that Rowling gave on July 8, 2000, with Anne Johnstone, The Herald (Glasgow):
"For me one of the big challenges was to make sure I knew the laws, both physical laws and the legal system within the wizarding world because until you know the boundaries, there's no tension."
One of her fundamentals is that you can't reverse death: "That's a given. Without it the plot would fall apart, though in Book Seven you'll see just how close you can get to the dead. You can be brought back from being petrified and from injuries that in the real world are mortal, depending on the degree of skill that a particular wizard possesses. You can't go to any wizard and say 'Will you cure my terminally ill relative?' It's a mirror image of the real world in that sense."
We saw petrified people brought back, and Harry has suffered enough injuries, if not to kill him, to certainly leave him permanently disabled; because of the rules she has set for magic in the books, Harry has come through some harrowing experiences. Dumbledore alludes to having sustained a life-threatening injury when he destroyed the ring Horcrux, and Harry foolishly used a curse he didn't understand, which resulted in serious injury to Draco--possibly it could have caused his death, but we weren't ever told that. Had it not been for Severus Snape's high "degree of skill", who seems to be the "particular wizard" to which she refers, Dumbledore, Draco and Katie Bell would likely have died.
But that is all different than "how close you can get to the dead". There are some great discussions going on at HogwartsProfessor (John Granger) and Sword of Gryffindor (Travis Prinzi), so I won't repeat all of that here. Much of the talk has turned to the Veil, and I certainly think that's a possibility. One other thing that keeps lurking at the back of my mind is that when they were filming POA, Cuaron wanted to throw in a graveyard, and Rowling told him, no, you can't put it there, because the graveyard is in some other location. So far, we still haven't seen a graveyard at Hogwarts, unless it's where Dumbledore's tomb is now. So coming near the dead could be in some graveyard as well.
Anyway, just go there and enjoy the discussions. We only have 19 more days for all the speculation; after that we'll be discussing canon and not our wild theories, which always seem to be the wildest just before we get the book in our hands.