I only heard about this from my boyfriend, I didn't actually see the interview myself. But here's what he told:
In the interview, Rowling showed that she already had almost finished book 7 completely! She got so many great ideas while she was writing the second book that she had to write them down and decided to use them for the last book. In the interview she showed a sketchbook full of notes to the camera, behind a glass of course so no-one could see what was written on it, and she said: "Oh, and any kdis who are watching this and want to get your hands on this, don't even think about breaking into my house. I have it well hidden in a safe place."
She also said that a lot of characters are going to die in the 7th book and that one of the deaths took her 3 months to write because it was so very gruesome. O.O So I think it's safe to assume the books get a lot darker as Harry grows up.
