Apparently outgoing showrunner Russell Davies envisioned something with Rowling being attacked by a space bug thing, and the world that lives inside of her head (with magic, and wizards and wands... oh my!) comes to life and the Doctor has to save her. Tennant didn't think this was a good idea. Seemed to think it was too spoofy, so the plan got scrapped. (You don't want to make the Doctor angry. You wouldn't like him when he's angry.) Now they've got a new idea or something. Whatever. I'm sure Rowling has better things to do... like count her money.

Personally, I'm kind of glad that the two worlds aren't overlapping. My little geeky brain would probably overload. I'd short-circuit and just start walking around the house waving my sonic screwdriver and yelling "winguardium leviosa." And really... no one needs to see that.