I recently looked up a lot of interviews online, and have saved them to my computer, but I haven't been able to read through all of them.
About the World Book Day chat being the only recent one...
well, it's not. I know for a fact Rowling has given interviews to a magazine called Movie Magic. She's been doing quite a bit of promoting, and with her, from the very beginning, she'd hint at something that will happen in a book much later. In one interview that I know she did, I read it in a magazine, and no, I don't remember which magazine, she said that someone unexpected would do magic. This was at least before OotP. She didn't say it would be in book 5, 6, or 7. So try looking in not-quite-so-recent chats... especially for the rumors that seem to be everywhere (they've had time to spread, so if she DID really say it, it would have been a while ago)
happy hunting.
